U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
The following discussion presents the rationale and supporting evidence for Handbook recommendations pertaining to these eight proven and promising practices.
A motorist's ability to use highway information from signing and delineation is governed by information acquisition, or how well the source can be seen. It is also governed by information processing, or the speed and accuracy with which the message content can be understood. When either of these key aspects of driver performance is compromised, the result is delayed decision making, erratic behavior, and maneuver errors.
Taylor and McGee (1973) investigated driver behavior at exit gore areas to determine the causes and characteristics of erratic maneuvers. Interviews were also conducted with many drivers whose actions at the gore area were indicative of route choice difficulties. Analyses of the patterns of erratic maneuvers (e.g., cross gore markings, cross gore area, stop in gore, back up, sudden slowing, lane change, swerve, stop on shoulder) and on-site driver interviews were used to determine causative factors of these maneuvers. The most frequent erratic maneuver was crossing the gore marking, which had a 69 percent relative frequency of occurrence for drivers exiting, and a 61 percent relative frequency of occurrence for drivers traveling through the interchange. Most of the motorists who made erratic maneuvers (77 percent) were unfamiliar with the route on which they were traveling. Interviews with exiting motorists who made erratic maneuvers indicated that more than half of the drivers were not adequately prepared for the exit. These drivers indicated that the signs lacked needed information or that the information was misleading. Interviews with drivers who made erratic maneuvers and continued through, indicated that approximately one-half had difficulty identifying their direction. Approximately 35 percent stated the signing was not clear, 21 percent responded that they could not clearly distinguish the location of the exit ramp, and 34 percent thought the road markings were inadequate.
|Applications in Standard Reference Manuals|
|MUTCD (2009)||AASHTO Green Book (2011)||NCHRP 500 - Volume 9 (2004)||Traffic Engineering Handbook (2009)|
|Sects. 2A.11, 2A.13, 2A.17, Sects. 2B.03, 2B.37 through 2B.41
Tables 2B-1 & 2C-4
Sects. 2C.14, 2C.15, 2D.05, 2D.06, Sects. 2E.10 through 2E.12 & 2E.14
Tables 2E-2 through 2E-5
Sects. 2E.19 through 2E.24
Figs. 2E-3 through 2E-16
Figs. 2E-22, 2E-26 & 2E-27, 2E-28, 2E-30, 2E-34 through 2E-40
Sects. 2E.31 & 2E.33 through 2E.37
Sects. 2E-40 through 2E-53
Figs. 3B-8, 3B-10, 3B-12, & 3B-23
Sects. 3C.01, 3C.03, 3D.01, 3D.02
Figs.6H-42 & 6H-43 plus accompanying notes
|Pg. 3-146, Final Paragraph
Pg. 10-71, Sect. on Signing and Marking
Pgs. 10-96 through 10-101, Sect. on Gores
Pg. 10-103, Final Paragraph
Pg. 10-113, Para. 3
|Pgs. V-15-V-17 Sect. on Strategy 3.1 B3: Increase Size and Letter Height of Roadway Signs (T)
Pgs. V-22-V-23 Sect. on Strategy 3.1 B8: Improve Roadway Delineation (T)
|Pgs. 372-373, Sect. on Sign Sizes
Pg. 391-392, Sect. on Older Drivers and Pedestrians
The following discussion of exit signing issues focuses on the legibility of text, the understandability of diagrammatic guide signs, and the placement of devices to provide needed message redundancy while avoiding information overload.
Prior to the Millennium edition of the MUTCD, legibility standards assumed that a 1-in tall letter was legible at 50 ft, which roughly corresponds to a visual acuity of 20/25; as documented in the Transportation Research Board's Special Report 218 (1988), this "legibility index" value of 50 ft/in exceeds the visual ability of 30 to 40 percent of drivers who are 65 to 74 years of age, even under favorable contrast conditions. The MUTCD (2003) section 2A.14 provided guidance for determining sign letter heights, which indicated that sign letter heights should be determined based on 1 inch of letter height per 40 ft of legibility distance. A 40 ft/in standard for signs can accommodate the majority of aging drivers if contrast ratios (between the legend and background) are greater than 5:1 (slightly higher for guide signs) and luminance is greater than 10 cd/m2 (candelas per square meter) for partially reflectorized signs. However, a more con-servative standard corresponding to 20/40 vision (i.e., a legibility index of 30 ft/in) would accommodate a greater proportion of aging drivers under a wider range of viewing conditions. The 2009 MUTCD (Section 2A.13) has updated the recommendation for letter heights accordingly, stating that a minimum specific ratio of 1 inch of letter height per 30 feet of legibility distance should be used.
Nighttime legibility requirements were addressed by Staplin, Lococo, and Sim (1990), who conducted a laboratory simulation using 28 young/middle-aged subjects (ages 19–49) and 30 older subjects (ages 65–80) to measure age-related differences in drivers' ability to read unique word combinations (of four letters) on green-and-white guide signs. As expected, older drivers required significantly larger letter sizes to read the (unfamiliar) words than younger drivers. Translating the 20-ft subject-to-stimulus distance in the laboratory to a requirement of 600 ft to read a freeway sign, the data showed that older subjects would require a letter height of 24 in, corresponding to an acuity of 20/46. This corresponds to a legibility index of 25 ft/in for positive contrast (lighter characters on darker background) highway guide signs.
In a review of State practices, McGee (1991) reported that Oregon reduced the size of letters on their freeway signs from 13.33 in uppercase and 10 in lowercase to 8 in and 6 in, respectively. They received numerous complaints that the signs were difficult to read at highway speeds and they therefore returned the letter sizes to their original heights (George, 1987). By contrast, North Carolina, in consideration of aging driver needs, increased the Interstate shield size from 36 in to 48 in, the uppercase letter size from 16 in to 20 in, and the lowercase letter size from 12 in to 15 in on guide signs at freeway-to-freeway interchanges (McGee, 1991). No evaluation was planned to determine the effectiveness of this countermeasure.
Garvey, Pietrucha, and Meeker (1997, 1998) conducted daytime and nighttime controlled field studies with aging drivers, to compare word legibility and word recognition distances obtained with the Standard Series E(M) font, and a new font with the proprietary name Clearview®, which was designed to reduce the effects of a phenomenon referred to as "irradiation," "halation," or "overglow." This phenomenon occurs when bright bold strokes bleed into a character's open spaces under headlight illumination, causing the lettering to appear blurry, rendering the text illegible. The study details are presented on page 152 in Chapter 7, Design Element 10 (Street Name Signs). Two versions of this experimental font were employed: one version matched Series E(M) in letter width and height, but because of its smaller intercharacter spacing, it resulted in a smaller word area (referred to in this Handbook as Clear 100); and one version contained letters that were increased in size to 112 percent of the standard font, so that the words created with the experimental font (referred to in this Handbook as Clear 112) were the same size as the standard font words. The signs were erected with a lateral offset of 12 ft from the center of the observation vehicle (or 6 ft) outside of the right edge line, and were raised to a height of 6 ft above the ground. Study results indicated that during the day, the Series E(M) fonts and both of the experimental fonts produced approximately equal reading distances. At night, using low-beam headlights and bright signing materials, the Clear 112 font (occupying as much sign space as the standard font) produced significantly longer legibility distances (22 percent longer) and recognition distances (16 percent increase) than the Series E(M) font. With highway-size signs on 55-mph roadways, this increased legibility could add up to two full seconds, or an additional 160 ft to the interval in which drivers must read and respond to a sign.
Hawkins,et al. (1999) conducted a study of the legibility of three sign alphabets with lowercase letters: the standard U.S.DOT/FHWA Series E(M); Transport Medium (the alphabet used in Great Britain for overhead guide signs with positive contrast); and Clearview®. The British Transport letters were designed in the 1950's to eliminate irradiation when viewed at nighttime using headlights (Hawkins et al., 1999), and consequently a narrower stroke width is used for positive contrast signs (white legends on a blue or green background) than is used for negative contrast signs (black legends on white, yellow, or orange backgrounds). Also, the letters are designed to fit on tiles, which are placed side-by-side to create words, eliminating the need to measure distances between words as is done in the U.S. Hawkins et al. indicate that the Clearview® font retains the visual proportions of the standard FHWA alphabets, but it also incorporates desirable attributes from foreign and domestic typefaces, particularly British Transport Medium. However, the Clearview® letter is typically wider than the same letter in Transport Medium.
Hawkins et al. (1999) employed 54 individuals to participate in the controlled field study as follows: 7 "young" drivers, age 35 or younger; 18 "young-old" drivers, ages 55 to 64; and 29 "old-old" drivers age 65 and older. Word legibility and word recognition data were recorded as an experimenter drove three subjects at a time toward the test signs, during daytime and nighttime. Each test sign was created using three, six-letter words arranged vertically on the sign, all in the same font, using high intensity white letters on green high intensity sheeting. Signs measured 12 by 9 ft and were either ground mounted or mounted overhead. The ground-mounted signs were erected at a height of 7 ft from the ground to the bottom of the sign, and were placed 30 ft from the right edge of the travel lane. The overhead signs were erected 20 ft above the traveled lane. Each word used a 16-in initial uppercase letter, followed by 5 lowercase letters. The lowercase letter size varied somewhat according to the alphabet type, but was generally 75 percent of the uppercase letter size (12 in).
Results indicated that the Clearview® font was more legible at both the mean and 85th percentile levels than the Series E(M) font for signs placed overhead, both under daytime and nighttime conditions. The 85th percentile daytime legibility index for the young-old drivers was 40, and for the old-old drivers it was 30 ft/in for the Series E(M) font. The extent of the improvement was in the range of two percent, however, some driver groups experienced improvements in legibility distance for the Clearview® font that were over nine percent greater than those experienced with the Series E(M) font. Hawkins et al. state that the improvement was greatest for drivers with poor vision (worse than 20/40). For the ground-mounted signs, Clearview® was less legible under daytime conditions than Series E(M), and only slightly (less than 2 percent) more legible at night. The Transport Medium font did not show improvements in legibility distance over the Series E(M) font.
In terms of recognition distance, for the overhead signs, the Clearview® font produced larger recognition distances than the Series E(M) font, in both daytime and nighttime conditions, except for the drivers with minimum (20/40) vision at night. The extent of the improvement in recognition distance was up to 8.7 percent—considerably higher than the improvements found in legibility distance, and translates to an increase in recognition distance of up to 50 ft. For ground-mounted signs, Clearview® produced a small improvement at night among worst-case drivers, but showed no improvement under daytime conditions. The 85th percentile recognition distance for drivers age 65 and older for ground-mounted signs at night with Clearview® font was 8.6 percent greater than that obtained with the Series E(M) font. Under a few conditions (85th percentile daytime distance for ground-mounted signs for all subjects, and for those age 65+), the Transport Medium font produced recognition distances that were, on average 3 percent greater than those obtained with the E(M) font. At nighttime, the 85th percentile recognition distance for drivers age 65 and older was 12.2 percent higher for Transport Medium font on the ground-mounted signs than for the Series E(M)font. The authors note that the recognition data showed significant variability from one condition to the next, and caution readers to take care in applying the results.
Overall, Hawkins et al. (1999), state that the results of their study indicate that the Clearview® font was more effective than the Series E(M) font in the overhead position, in both daytime and at night, with the greatest improvement achieved for the worst-case drivers. They further indicate that while the Clearview® font may be more appropriate than the E(M) font for overhead signs, ground-mounted signs should continue to use only the Series E(M) alphabet. Hawkins et al. suggest using different design parameters for overhead and ground-mounted signs to account for the differences in performance characteristics of each. They also note that the Clearview® font is an evolving font, and that there are differences in the font used in the Garvey et al. (1997) study and in the current research.
In Knoblauch, Nitzburg, and Seifert's (1997) focus group discussions with aging drivers, participants indicated that they prefer overhead signs to those mounted on the side of the road, stating a need for redundant (overhead) signs to provide advance notice of upcoming exits, including the distances to each, and indicating whether the exit is on the right or left side of the highway. This report, coupled with findings that the Clearview® font provides greater recognition and legibility distance over the standard Series E(M) font when used on overhead signs under both daytime and nighttime conditions, identifies highway destination signs (D1-D3) placed over the highway and fabricated using the Clearview® font as the preferred practice to accommodate aging drivers.
A more recent project was conducted to compare the nighttime legibility distance of the Clearview® font (Clearview® Regular Express) to that of the standard series E(Modified) font on overhead and right shoulder-mounted freeway guide signs, fabricated with either the standard ASTM Type III sheeting or with microprismatic sheeting (Carlson, 2001; Carlson and Brinkmeyer, 2002; and Carlson and Hawkins, 2003). Two types of microprismatic sheeting were evaluated: ASTM Type VIII and ASTM Type IX. Participants in this closed-course field study consisted of 20 young drivers (ages 18-34), 20 middle-aged drivers (ages 35-54), and 20 older drivers (age 55+), with 10 males and 10 females in each group. Each had a valid driver's license, and their acuity ranged from 20/10 to 20/50. Test vehicles included a 2001 Chevy Suburban with tungsten-halogen replacement bulb headlights and a 1989 Ford Crown Victoria, LTD with sealed-beam headlamps. Signs were made using 16-in uppercase letters with 12-in lowercase letter heights. All words were presented on a sign background 12-ft wide by 9-ft tall. Sign legends and backgrounds were made with the same type of retroreflective sheeting. The bottom of each overhead sign was 18 ft above the road surface. The bottom of each shoulder-mounted sign was 9 ft above the road surface with an offset of 24 ft from the edge of the right travel lane to the left of the sign background. Study was conducted on a closed-course facility in a dark rural area with no ambient lighting. Test subjects drove each vehicle type on a runway at night under dry conditions, beginning at a distance where the signs were not legible. They accelerated to 35 mph, set the cruise control, and concentrated on reading the test word on a sign. When the subject read the word correctly, a researcher in the vehicle recorded the distance. Each subject read 56 randomly selected test words equally distributed between the Clearview® Series and the Series E(M) alphabets. Of the 56 words, 40 were located in the shoulder-mounted position and 16 were in the overhead distance. Study findings overall indicated that the combined benefit of microprismatic sheeting and Clearview® font resulted in an increased legibility distance of approximately 75 ft, and were greatest for drivers age 55 and older. Detailed results by sign location are provided below.
For shoulder-mounted signs, the overall mean legibility distance associated with Clearview® was 32 ft greater (a 5.2% improvement) than for Series E(M). Improvements in mean legibility distances ranged from 18 to 58 ft, with the largest differences occurring with the sealed beam headlamps and the Type IX sheeting. Assuming a 70 mph roadway, the improvements would result in added time to read a sign between 0.2 and 0.6 sec; for a 55 mph roadway, improvements would range from 0.2 to 0.7 sec. The increased legibility distances associated with the Clearview® alphabet were statistically significant. The largest difference between the Clearview® and the Series E(M) alphabets were associated with aging drivers, where legibility differences were 6.0 percent longer with the Clearview® than the Series E(M). This difference equates to an additional 0.45 sec of reading time for aging drivers. Legibility differences were 5.8 percent and 4.6 percent greater for the Clearview® than the Series E(M) font, for young and middle-aged drivers respectively. The effect of age was significant, as were the main effects of alphabet and sheeting. The interaction between age and alphabet was not significant, however.
For overhead guide signs, overall mean legibility distances were 53 ft greater for the Type IX sheeting than for the Type III sheeting (a 9.5% improvement). Type VIII sheeting was not tested on overhead signs, but would be expected to provide longer legibility distances than Type IX sheeting. The effect of sheeting type was significantly significant, with legibility distances significantly greater with Type IX sheeting than for Type III sheeting. Luminance values and legibility distances were significantly greater for the sealed beam headlights than for the tungsten-halogen bulbs. For Type IX sheeting, the overall mean legibility distance for the Clearview® alphabet was 40 ft greater than for the Series E(M) alphabet (a 6.7% improvement). The main effect of age was significant, as was the interaction between age and alphabet. As age increased, the benefits of Clearview® were more pronounced. For the aging drivers, Clearview® provided an increase in legibility distance of 33 feet, or 6.8% (compared to increases of 2.3% and 3.5% for young and middle-aged drivers). The increased legibility distance results in an additional 0.33 seconds of reading time, assuming a 70 mph) highway.
A legibility analysis using the data collected in the field within the ERGO program showed the sequential and overall benefits in legibility distance expected by switching from Type III sheeting to microprismatic sheeting and by switching from Series E(M) alphabet to Clearview® alphabet. For overhead signs, switching from Type III sheeting with Series E(M) to microprismatic sheeting with E(M) increases mean legibility distance by 44 ft. Switching from microprismatic sheeting with E(M) to microprismatic sheeting with Clearview® adds another 30 ft. Together, switching from Type III with Series E(M) to microprismatic with Clearview® would increase legibility distance by 70 ft, an 11.9 percent improvement. Assuming a 70 mph highway, the overall legibility improvement provides drivers with an extra 0.68 s to read an overhead guide sign (and for 55 mph it is an extra 0.86 s). Assuming a last look distance equal to 3 s before passing the sign, these time improvements equate to a 24.4 percent increase in time to read an overall guide sign at 70 mph and an increase in 21.2 percent on a 55 mph highway. For shoulder-mounted guide signs, switching from Type III sheeting with Series E(M) to microprismatic sheeting with E(M) increases mean legibility distance by 41 ft. Switching from microprismatic sheeting with E(M) to microprismatic sheeting with Clearview® adds another 33 ft. Together, switching from Type III with Series E(M) to microprismatic with Clearview® would increase legibility distance by 74 ft, a 12 percent improvement. Assuming a 70-mph highway, the overall legibility improvement provides drivers with an extra 0.72 s to read an overhead guide sign (and for 55 mph it is an extra 0.92 s). Assuming a last look distance equal to 3 s before passing the sign, these time improvements equate to a 24.1 percent increase in time to read an overall guide sign at 70 mph) and an increase in 19.8 percent on a 55 mph highway.
FHWA issued an Interim Approval for the optional use of the Clearview® font for positive contrast legends on guide signs on September 2, 2004. The conditions of Interim Approval are specified at http://mutcd.fhwa.dot.gov/res-ia_clearview_font.cfm.
Carlson and Holick (2005) conducted a controlled field study to determine how the legibility of full-scale unlit guide signs could be maximized with combinations of font and retroreflective sheeting. A total of 30 subjects participated, distributed equally across three age groups (ages 18 to 34; 35 to 54; and 55+), with equal numbers of males and females in each age group. Two fonts were studied: Series E (Modified) and Clearview® 5WR, where "R" stands for "reduced," designed to produce the same word lengths on a sign as Series E (Modified). Clearview® 5WR has letter spacings reduced by 6.4 percent from the standard Clearview® 5W. Five combinations of retroreflective sheeting were studied:
Two luminance levels were studied: low-beam headlamps at full power and lowbeam headlamps at 27 percent power, to simulate the possible ranges in sign luminance that could result from different conditions such as sign position (over-head or shoulder mounted), signs viewed from heavy trucks, and vehicles with poorly maintained or misaimed headlights. These levels corresponded to 13.0 and 3.6 cd/m2 for the white Type III materials at 640 ft, based on research showing a 50th percentile minimum required luminance of 3.4 cd/m2 for a legibility index of 12.2 m per 40 ft/in of letter height.
All guide signs used in the Carlson and Holick (2005) study had white 16-in uppercase letters on a green background. The lowercase letters were the appropriate size, depending on the font used. Half of the signs were constructed with Series E(Modified) font and half with Clearview® 5WR font. The signs were posted to follow current signing practices as closely as possible (9 ft above the road surface and 30 ft from the edge of the travel lane). The study was conducted on a closed-course facility in a dark rural area with no ambient lighting. Subjects drove a 2000 Ford Taurus sedan, with an experimenter in the front passenger seat who (unaware to the subjects) controlled the headlamp illumination level (to produce two levels of sign luminance throughout the study). Subjects were told to read the word on the guide sign as soon as they could correctly identify it, and that there was no penalty for being wrong.
The effect of font on legibility distance was significant, with the average legibility distance for Clearview® 5WR 593 ft compared to 570 ft for Series E(Modified). In a smaller and independent study conducted within this study, Clearview® 5W was compared to Clearview® 5WR, and the two fonts produced statistically equivalent results, with the legibility of Clearview® 5W equal to 593 ft and the legibility of Clearview® 5WR equal to 590 ft compared with Series E(Modified), which produced an average legibility distance of 539 ft.
For each age group studied by Carlson and Holick (2005), all combinations of sheeting produced higher legibility distances compared to Type III on Type III. Sign brightness was statistically significant and was most evident for the aging subjects. The mean decrease in sign legibility from the full headlight to the reduced headlight condition was 8.5 percent for the young group, 3.3 percent for the middle group, and 16.7 percent for the older group.
All signs with microprismatic legends performed significantly better than signs with Type III legends. There were no significant differences in legibility distance between signs made with microprismatic legends. Signs made with microprismatic backgrounds performed statistically similarly to signs made with high-intensity backgrounds.
The longest legibility distances were achieved with a microprismatic legend on a microprismatic background, but the legibility distances were not statistically different from those achieved with microprismatic legends on Type III backgrounds. The study authors state that this is important because of the cost of the microprismatic materials ($3.00 to $4.00 per sq/ft) compared to the cost of Type III materials ($1.00 to $1.50 per sq/ft.)
Collapsing data across subject age and luminance level, and using only Clearview® font (based on its significantly longer legibility distances), the legibility distances are shown in Table 41 for the retroreflective sheeting types. Combinations within the two groupings (* or x) had statistically similar legibility means.
|Legend Sheeting Type||Background Sheeting Type||Legibility Distance||Significance Grouping|
|(legibility means with the same symbol are not significantly different from each other)|
|IX||IX||595.4 ft||* X|
|VIII||III||591.5 ft||* X|
Caution must be taken with the finding by Carlson and Holick (2005) that legibility distance was not affected by background sign material, considering the small sample size within each group. Although the study authors recommend the use of Type II backgrounds over microprismatic backgrounds as a cost-saving measure, earlier findings by Carlson and Hawkins (2003) provide a rationale to first recommend the use of microprismatic materials for both the legend and the background, and then allow for the use of Type III materials for the background if cost is an issue. Carlson and Hawkins (2003) recommended that microprismatic sheeting be used on overhead signs in place of encapsulated sheeting, as a countermeasure for the increasing number of SUVs on the road with reduced headlight illumination on signs. In that study, the legibility distances were smaller for drivers in the Subaru (with larger observation angles, tungsten-halogen bulbs, and reduced light directed toward the sign) than the LTD (a passenger car with smaller observation angles, sealed-beam headlamps, and increased light directed toward the sign).
Moving from a consideration of legibility issues to the broader question of how well a motorist can actually use highway sign information, reading time and ease of recall for sign messages deserve attention. Reading time is the time it actually takes a driver to read a sign message, contrasted with exposure time or available viewing time, which is the length of time a driver is within the legibility distance of the message. As drivers travel, they must look away from the highway to read signs posted overhead or at the side of the road, and then back to the roadway. During each glance, the maximum amount of text that can be read is three to four familiar words or abbreviations. A motorist's rapid understanding and integration of message components in memory will greatly assist his/her recall of the message while deciding upon a response. Two errors in message presentation must be avoided: (1) providing too much information in too short a time and (2) providing ambiguous information that leaves either the intent of the message or the desired driver response uncertain.
Mace, Hostetter, and Seguin (1967) conducted laboratory, controlled field, and observational field studies to evaluate how information presentation time (the amount of time that a sign is readable to a driver) and information lead distance (the distance from an exit that the advance sign is placed) affect exiting behavior at freeway interchanges. They found that 0.25 mi is inadequate for information lead distance and, because there were few differences in driver exiting behavior with information lead distances of 0.5 mi and 1.0 mi, that 0.5 mi is optimal. In addition, a viewing time of 5 s was adequate for signs containing one to four pieces of information. Lunenfeld (1993) noted that a driver's short-term memory span is between 0.5 and 2 min, and that drivers may forget advance interchange information messages if the time span between the advance notification and the exit ramp exceeds the memory limit. He advocates the use of repetition for interchange information treatments (multiple/successive signs), which will also aid in situations where a sign is blocked by foliage or trucks.
The effect of diagrammatic signing on driver performance at freeway interchanges was studied by numerous researchers in the early 1970's. Bergen (1970) found that graphic guide signs permitted significantly better route guidance performance than conventional signs on certain interchanges, such as collector-distributor with lane drop and multiple split ramps. In pilot studies conducted in New Jersey, Roberts (1972) found that diagrammatic signs that included lane lines were more effective (resulted in a significant reduction in erratic maneuvers) than conventional signs at the interchange of I-287 and U.S. 22, a complex interchange with both left- and right-side exits. Flener (1972) commented on the difficulty in evaluating the effectiveness of traffic control devices in reducing erratic maneuvers at exit gore areas using before and after designs, due to the "novelty effect." Although Roberts (1972) noted that the change could be attributed to the greater attention-getting value of novel signs, it was demonstrated that diagrammatic guide signs provide advance information that is readable at a farther distance than that provided by conventional sign text, as well as information about the number of lanes available for any one movement.
Roberts, Reilly, and Jagannath (1974) studied the effectiveness of diagrammatic versus conventional guide signs in a field study at 10 sites. The results were mixed. Several sites showed a reduction in stopping, backing, or weaving erratic maneuvers after installation of the diagrammatic signs. Some sites showed a reduction in stopping and backing maneuvers but an increase in weaving maneuvers (or vice versa). Still other sites showed no change as a function of sign type. Stopping and backing erratic maneuvers were reduced, however, at 9 of the 10 sites.
Taylor and McGee (1973) noted that the main advantage of diagrammatic signing lies in the ability to provide information regarding the interchange layout prior to the exit area. Sign format, however, remains an issue. Conflicting evidence on the effectiveness of diagrammatic signs was reported by Gordon (1972), who found that conventional signs produced fewer lane-placement errors and errors on exit lanes and were more quickly responded to than experimental diagrammatic signs tested at six interchanges in a laboratory study. At the same time, an analysis of particular diagrammatic designs showed that when a diagrammatic sign provided a single arrow or a forked arrow, reaction time was faster and there were fewer errors compared with the conventional sign. Zajkowski and Nees (1976) studied subject response time and correctness of lane choice as a function of sign type, in the laboratory. They found that response times were consistently longer for diagrammatic signs than for conventional signs; however, the difference may have been attributable to an increase in information on diagrammatic signs. There were more correct lane-choice responses for conventional signs, and subjects reported more confidence in their lane-choice decisions and a preference for conventional signs. Mast, Chernisky, and Hooper (1972) found that some drivers may require more time to read and interpret information on diagrammatic signs in comparison with conventional signs, and driver information interpretation time may increase as the graphic component of the sign becomes more complex.
Aging drivers participating in focus group discussions have indicated that they prefer large, lighted overhead signs with arrows that indicate the lanes for specific destinations, especially if they are approaching a fork in the road (Knoblauch, Nitzburg, and Seifert, 1997). These aging drivers stated that the arrows on the overhead signs do not always point to the correct lane, causing drivers to change lanes needlessly; they also stated a desire to see signs that indicate when two travel lanes bear to the same destination.
Brackett, et al. (1992) conducted a survey of 662 drivers in three age groups (younger than age 25, ages 25-54, and 55 and older) comparing alternative methods of providing lane assignment information on freeway guide signs. The findings of several comparisons in the research are reported, although no analyses using age as an independent variable were performed. First, when two common routes were displayed side by side on an exit guide sign, approximately one-half of the drivers believed that the destinations referred to different routes to be accessed by different lanes (i.e., drivers spatially cluster information with each arrow, assuming that information located on the left side of a sign is associated with an arrow also on the left side, and information on the right side is associated with EXIT ONLY or EXIT ONLY with an arrow). When destinations were arrayed one below another, 85 percent of the drivers understood that they were a common route. Second, white downward arrows used in a side-by-side format with an EXIT ONLY (E11-1) panel to indicate that two lanes could exit, were misunderstood by 80 percent of the subjects. Third, 56 percent of drivers misinterpreted the phrase NEXT RIGHT on conventional signs as an indication of a mandatory exit, and 30 percent misinterpreted the phrase NEXT LEFT in the same manner, when these signs were placed over the right and left lanes, respectively. Fourth, when conventional MUTCD diagrammatic signs were compared with modified diagrammatic signs that provided separate arrows for each lane, the modified diagrammatic signs resulted in a 13 to 17 percent greater understanding of when a lane must exit and when an adjacent lane may exit or continue through (two-lane exit with optional lane). When the number of arrow shafts exceeded the number of lanes (for example, when there is an auxiliary exit lane downstream of the overhead sign), less than 30 percent of the respondents understood that there would be an added exit lane downstream on the right. With one arrow per lane, comprehension increased by 28 percent over when there were more arrows than lanes (optional use or added lanes). Figure 92 display an example of a conventional diagrammatic sign (from Section 2E.20 of the 2003 MUTCD) and a modified diagrammatic sign for this exit situation. The sign in Figure 92b resembles the recommended sign shown in Figure 2E-3 of the 2009 MUTCD; Figure 92a is similar to the sign shown in Figure 2E-7 of the 2009 MUTCD.
Figure 92. Example of MUTCD diagrammatic sign (a) and modified diagrammatic sign (b) used in comprehension evaluation (Brackett, Huchingson, Trout, and Womack (1992))
The benefit of using one (upward-pointing) arrow per lane to show the number and direction of lanes for a given highway geometry on freeway guide signs was demonstrated by Golembiewski and Katz (2008). Twenty-four younger (mean age = 33) and 24 older (mean age = 79) drivers viewed five alternative diagrammatic sign designs at the Highway Sign Design and Research Facility at the FHWA Turner-Fairbank Highway Research Center. As the signs were presented to participants, they indicated by a button press when they were sure of which lane(s) could be used to get to their destination. The distance to each sign when the choice was made (decision sight distance) and the correctness of each decision were recorded.
The five designs evaluated in this study included 1) Standard, which followed the 2003 MUTCD guidance for freeway guides signs at lane splits; 2) Modified, which followed the same guidance but included Exit Only placards where appropriate; 3) Enhanced, which followed the MUTCD guidance but had wider dashed lines and wider arrowheads; 4) Enhanced Modified, which was similar to Enhanced, but included Exit Only placards when appropriate; and 5) Arrow Per Lane, which used upward pointing arrows centered over each lane to indicate movements appropriate for that lane. All study participants viewed all sign designs in a repeated-measures design, counterbalancing the order of presentation across subjects.
Signs using the Arrow Per Lane design yielded significantly better performance for aging drivers than the other types. While the performance of younger participants was significantly better than that of the older participants they, too, benefited from this design. These findings indicate that the Arrow Per Lane sign type is appropriate for all drivers and is especially beneficial for aging drivers. Golembiewski and Katz (2008) conclude that, because signs in this experiment were presented without the surrounding roadway context, the present study may even underestimate the benefit of this design relative to the others evaluated.
Violations of driver expectancy, use of alcohol, and reductions in the ability to integrate information from multiple sources to make navigation decisions while concurrently controlling the vehicle may all result in driver confusion at critical decision points, resulting in wrong-way maneuvers. Tamburri and Theobald (1965) found that many aging drivers and drinking drivers did not know where their wrong-way movement began (i.e., they could identify neither where the decision point was nor the location of the wrong-way maneuver). The information provided for this design element focuses on positive signing and ramp design for freeway entrances, as well as the use of advanced diagrammatic guide signs on urban multilane arterials leading to freeways, while design element 30 focuses on wrong-way signing and pavement markings at interchanges.
Age-related diminished capabilities contributing to wrong-way movements include the cognitive capabilities of selective attention and divided attention, and the sensory/perceptual capabilities of visual acuity and contrast sensitivity. Selective attention refers to the ability to identify and allocate attention to the most relevant targets in the driving scenario on an instant-to-instant basis, while divided attention refers to the ability to perform multiple tasks simultaneously. Individuals less capable of switching attention, or who switch too slowly, may increase their chances of choosing the wrong response or choosing the correct response too slowly. Treat, et al. (1977) reported that 41 percent of crashes in which aging adults were involved were caused by a failure to recognize hazards and problems, and that 18 to 23 percent of their crashes were due to problems with visual search. The selective attention literature generally suggests that for adults of all ages, but particularly for aging drivers, the most relevant information must be signaled in a dramatic manner to ensure that it receives a high priority for processing in situations where there is a great deal of complexity at the level of information to be processed.
In their study of highway information systems, Woods, Rowan, and Johnson (1970) found that motorists frequently experience difficulty in locating entrance ramps to freeways, and drivers were often confused when there were several side roadways intersecting in close proximity to the interchange area. These researchers suggested that more efficient use could be made of "positive" signing techniques in guiding motorists to the freeway entrance ramps and discouraging drivers from possible wrong-way maneuvers.
|Applications in Standard Reference Manuals|
|MUTCD (2009)||AASHTO Green Book (2011)||NCHRP 500 – Volume 9 (2004)||Traffic Engineering Handbook (2009)|
|Sects. 2A.23, 2B.37, & 2B.38
Tables 2B-1 & 2C-1
Sects 2D.45, 2E.52, 2B.41, 2E.53, 3B.20, 3D.01
Figs. 3B-24, 2B-18, 2B-19
Figs. 2D-11 through 2D-16
|Pgs. 10-83 through 10-87, Sect. on Wrong-Way Entry||Pg. 23, Paras. 1-2||Pgs. 372-373, Sect. on Sign Sizes
Pg. 391-392, Sect. on Older Drivers and Pedestrians
Woods et al. (1970) indicated that positive signing which indicates the correct path or turning maneuver to the motorist rather than a restriction may help most to minimize driver confusion at freeway interchanges. Examples include route markers, trailblazers, and a FREEWAY ENTRANCE sign that positively designates an entrance to the freeway. The California Standard specifies that large FREEWAY ENTRANCE signs (48 in x 30 in) be placed on on-ramps, but the location of the sign package (FREEWAY ENTRANCE sign, plus route shield, cardinal direction sign, and down diagonal arrows) should not be controlled by the use of the larger signs; smaller signs (36 in x 21 in) may be used for proper placement, if necessary.
Advance guide signs on multilane highway approaches to an interchange provide drivers with information about which lane they should use to enter the freeway in the desired direction of travel. Interchanges may require right turns from the multilane arterial for both available directions of travel on the freeway (cloverleaf interchanges), a left turn to travel left and a right turn to travel right (diamond interchange), or other combinations of right and left turns. This lack of consistency coupled with late recognition of the required entrance ramp lane may result in last-second lane-change maneuvers that increase the potential for crashes. Advance information is important for preventing erratic, risky lane change maneuvers during the approach to the interchange. Although it is desirable to present this information on overhead guide signs, it is costly and not always feasible. Zwahlen, et al. (2003) evaluated the effectiveness of ground-mounted diagrammatic guide signs on urban multilane arterials leading to freeways as a lower-cost alternative to overhead span-type sign bridges. In this field study conducted in actual traffic, 40 drivers (20 males and 20 females) who were unfamiliar with the test city area (Columbus, OH) drove an instrumented vehicle and were accompanied by an experimenter who provided instructions. The average age of the participants was 20 years old, and acuity was described by the researchers as "good." Ground-mounted diagrammatic signs were placed 0.5 mi and 0.25 mi along urban multilane arterials in advance of 6 highway interchanges before the last point of the gore (see Figure 93). The letter height for the destination letters was 6 in for uppercase letters and 4.5 in for lowercase letters. The letter height for the cardinal directions was 6 in (152 mm). The FHWA standard alphabet was used. The arrows were 8 in wide by 90 to 104 in long. All signs were fabricated with microprismatic Stimsonite 6200 traffic signing material (meeting ASTMD 4956 Type III and IV. These signs supplemented existing trailblazer assemblies on entrance ramps.
Figure 93. Ground-Mounted Diagrammatic Guide Sign for Urban Multilane Arterial, Used by Zwahlen et al. (2003)
Subjects in the Zwahlen et al. study began in a parking lot and were asked to start driving and to merge with traffic on a multilane arterial approximately 3 mi in advance of the test interchange. The experimenter told subjects to find their way to a specific freeway entrance ramp (i.e., I-270 Northbound); subjects were always started out in the wrong lane for accessing the freeway in the direction given by the experimenter. The measure of effectiveness was the distance from the point at which the subject realized he or she was in the wrong lane and a lane change was required, to the interchange gore. The study was performed at night to avoid the daytime delays associated with the freeway interchanges. Twenty-one subjects participated before the advanced diagrammatic signs were installed, and 19 drivers participated after installation of the signs. Subjects who participated in the "before" condition did not participate in the "after" condition. Overall, the "after" condition (diagrammatic guide signs present) was associated with a substantial, statistically and operationally highly significant improvement in lane change distance. Combining all six sites, the 50th percentile lane change distance in the "before" condition was 1,237 ft and in the "after" condition 2,686 ft. The 85th percentile lane change distance in the "before" condition was 666 ft and in the "after" condition 1,971 ft. These distances represent an increase in lane change distance by a factor of 2.2 and 3.0 between the before and after conditions. With ground-mounted diagrammatic guide signs, the test subjects (who were unfamiliar to the area) were able to initiate a lane change from the incorrect lane much earlier (4 to 5 times earlier at some interchanges) than in the period before the signs were installed. Although aging drivers were not represented in the sample, it is logical that they would benefit from upstream, redundant signing using positive guidance principles.
Aside from difficulties in the use of signs, problems for aging drivers at interchanges most likely to result from (age-related) deficits in spatial vision relate to the timely detection and recognition of pavement markings and delineation. Data from a study by Blackwell and Blackwell (1971) show that between age 20 and age 70, aging directly reduces contrast sensitivity by a factor of about 3.0. Mace (1988) stated that age differences in glare sensitivity and restricted peripheral vision coupled with the process of selective attention may cause higher conspicuity thresholds for aging drivers. Overall, these deficits point to the need for more effective and more conspicuous signing and delineation.
Vaswani (1974) identified specific sources of wrong-way movements where alcohol was believed not to be a factor. In this study, exit ramps on partial interchanges generated wrong-way maneuvers because, unlike the ramps on full interchanges that converge with right-hand traffic, the ramps meet the crossroad at about 90 degrees to accommodate both left and right turns. Therefore the wrong-way entries consist of left turns off of the exit ramp into wrong-way traffic on a two-way divided highway, right turns from the divided highway into traffic exiting the ramp, and left turns from the crossroad into the exit ramp. At intersections with four-lane divided highways (divided arterial and primary highways), 45 percent of the wrong-way entries were at their intersections with exit ramps or secondary roads. The wrong-way entries were due to left-turning vehicles making an early left turn rather than turning around the nose of the median. Almost all these crashes involved sober drivers.
Some ramp designs are more problematic than others. In Tamburri and Theobald's 1965 analysis of 400 wrong-way incidents where entry was made to the freeway via an off-ramp, the trumpet interchange category had the highest wrong-way entry rate, with 14.19 incidents per 100 ramp-years, and the full cloverleaf interchanges had the lowest wrong-way entry rate, with 2.00 incidents per 100 ramp-years. Parsonson and Marks (1979) also determined that several ramp types were particularly susceptible to wrong-way movements, as follows: half-diamond (3.9 per month), partial cloverleaf ("parclo") loop ramp (11.0 per month) and parclo AB loop ramp (6.7 per month). The parclo loop ramp and the parclo AB loop ramp share the same problem, which is an entrance and exit ramp in close proximity. The half-diamond is susceptible because it is an incomplete interchange, and drivers may make intentional wrong-way entries. A "problem" ramp has been defined as one that experiences more than five wrong-way movements per month; a corrected ramp has less than two per month (Rinde, 1978).
Vaswani (1974) found that on almost all the interchanges on which wrong-way entries had been made into the exit ramp or from the exit ramp onto the crossroad, the corner of the exit ramp flared into the right pavement edge of the crossroad. He suggested that such a flare provides for a very easy but incorrect right-hand turn, and may help to induce a driver to make a wrong-way entry from the crossroad into the exit lane. A countermeasure consisting of a sharp right-hand junction would require a driver to reduce speed and almost come to a stop before maneuvering into the left lane, and would also reduce the chances that a driver exiting the ramp would turn left into wrong-way traffic on the crossroad. Site inspections showed that where the flare was not provided and the left lane of the exit ramp and the passage through the median were channelized, no wrong-way entry to or egress from the exit ramps was reported. Additionally, Vaswani (1974) reported that generous widths of an exit ramp with its junction with the crossroad make wrong-way entry or egress from the exit ramp easy. Narrow pavement widths will discourage such entries. A serious impediment to turning maneuvers by heavy vehicles could also result from this strategy, however.
Vaswani (1974) also indicated that too large a set-back of the median noses from the exit ramp increases the width of the crossover and makes the intersection harder to "read." Vaswani suggests that if the width cannot be reduced, then pavement nose markings in the form of a striped median should be applied, for improved visibility of this design element. See also the discussion on page 122 of this Handbook for design element 5, about extending delineation treatments from a set-back median nose to the intersecting roadway.
The following discussion of exit ramp gore delineation focuses on studies conducted to determine which treatments are necessary to ensure rapid and accurate detection of the gore location and ramp heading, particularly under nighttime or reduced visibility conditions.
Taylor and McGee (1973) reported that the location of the gore is usually perceived easily during daylight hours, because a driver can rely on a direct view of the geometry, as well as signing and delineation. However, this task becomes considerably more difficult during darkness, because the driver can no longer rely on a direct view of the geometry, and exit gore signing may be misleading because of the inconsistency in the distance at which it is placed from the nose of the gore area from location to location. At night, delineation is probably the most beneficial information source to the exiting motorist, because it outlines and therefore pinpoints the location of the gore.
|Applications in Standard Reference Manuals|
|MUTCD (2009)||AASHTO Green Book (2011)|
|Sects. 2C.09 & 3F.03||Pg. 8-17, Paras. 1-2
Pgs. 10-89 through 10-101, Sect. on General Ramp Design Considerations
Taylor and McGee (1973) measured the effects of the presence of gore area delineation on driver performance at night, to determine which of various delineation devices (pavement markings, post delineators, raised pavement markers (RPMs), and a combination of treatments) were most effective. Measures of effectiveness included the point of entry into the deceleration lane, the exiting speed, and any erratic maneuvers. Two right-hand exits, one with a parallel-lane type of deceleration lane and one with a direct-taper type, were selected as test sites. Specifically, the treatment conditions were: (1) post delineator treatment—yellow post delineators placed along the ramp edge of the gore area, plus white delineators positioned along the through side; (2) RPM treatment—yellow RPM's placed on the ramp side of the gore (paint) markings, plus white RPM's on the through side; and (3) combination treatment—the post delineator treatment and the RPM treatment installed in combination.
The baseline condition for this study was moderately worn painted diagonal gore markings and edge lines, with no other delineation devices. All three delineation treatments produced earlier points of entry into the deceleration lane than under the baseline condition. The RPMs were more effective than the post delineators and produced earlier exiting points. The earliest exiting points were found with the combination of RPM's and post delineators. Gore area delineation reduced the frequency of erratic maneuvers at night at both sites. The RPM technique and combination treatment produced significantly lower exiting speeds than did the use of post delineators at one site, and all three treatments produced lower exiting speeds compared with the baseline condition.
The work by Taylor and McGee (1973) also included a comprehensive review of several case studies. As a result of their state-of-the-art summary, coupled with the results of their field observations in the study outlined above, a set of recommendations was developed for pavement marking delineation, post delineators, and RPMs; these recommendations, which have since been widely implemented, are described below.
For pavement marking delineation:
8- to 12-in wide white lines should be used to outline the exit gore, and where additional emphasis is necessary, diagonal or chevron markings are recommended.
An 8-in wide line with a 5-ft mark and 15-ft gap should be used as an extension of the mainline right edge line (or median edge line for left exits) and should replace the lane line for at least 1,000 ft upstream from the gore nose at an exit lane drop.
For post delineators:
Post delineators should be placed in the gore area to enhance nighttime visibility. White delineators are recommended for the through roadway side, and yellow delineators should be used on the exit side. A spacing of 10 to 20 ft, depending on ramp divergence angle, is recommended.
Yellow delineators should be placed along the right edge of the deceleration lane at a spacing of 100 ft. Beyond the beginning of the gore, the spacing is dependent on the degree of curvature.
White delineators should be placed on the inside shoulder of the through roadway, at a spacing of 100 ft, to help strengthen the through-way delineation in the exit area.
Raised pavement markers are recommended as a supplement to standard gore pavement markings and should be placed inside the "V" formed by the pavement marking lines.
Raised pavement markers should be supplemented with post delineators where the view of the roadway is limited, such as at vertical sections.
Other researchers have also evaluated the effects of RPM's at exit gore locations. RPM's have been shown to reduce erratic maneuvers through (painted) gores at exits and bifurcations.
Hostetter, et al. (1989) conducted a controlled field study using 15 subjects ages 18 to 60+, to determine the effect of lighting, weather, and improved delineation on driver performance. Data were obtained on two exits in dry and wet weather under full lighting with baseline delineation (see diagram in Recommendation 20 (A1)). The baseline system is similar to the delineation used at many of the partially lighted interchanges cataloged by the study authors during site selection, and in the opinion of an expert panel convened during the research, constituted a minimum system for partially lighted interchanges. Data were then obtained under partial lighting, with baseline and three improved delineation systems.
Upgrade 1 investigated by Hostetter et al. (1989) differed from the baseline in the use of RPM's along the left ramp stripe, and the substitution of fully retroreflective posts (46-in strip of 3-in wide sheeting) for partially retroreflective posts (18-in strip of 3-in wide sheeting) in the physical gore. Upgrade 2 differed from the baseline in the deployment of additional posts along the left ramp shoulder to create a spacing of 50 ft rather than 100 ft and in the installation of wide RPM's ("traffic diverters") on the gore strips to replace the 4-in RPM's placed adjacent to the gore stripes in the baseline system. Upgrade 3 replaced all baseline system partially retroreflective posts with fully retroreflective posts except in the gore, used RPM's along the left ramp stripe, and used beaded profiled tape containing a raised-diamond pattern for gore striping. The tape was used because it would project above a film of water during rain. The test sites were a half-diamond interchange and a full diamond, which contained very little ramp curvature. The exit ramps were 14 ft wide, with a single lane widening to two lanes near the intersection with the crossing roadways. Measures of effectiveness included ramp and spot/trap vehicle speeds, overall travel time, deceleration estimates, and lane placement, as well as selected types of erratic maneuvers and brake and high-beam headlight activations.
Analysis of delineation effects on ramp and spot speeds, and on speed distributions showed few differences under dry conditions. Under rainy conditions, effects were stronger but were neither large enough nor consistent enough to recommend improved delineation over the baseline system. Although Upgrade 3 produced fewer edge line encroachments under both dry and wet conditions, from the standpoint of operations, safety benefit, or cost-effectiveness, the upgrade did not demonstrate enough advantage to merit a recommendation for use on diamond interchanges with little ramp curvature.
Lerner, et al. (1997) conducted a series of laboratory and field studies to identify conspicuity and comprehensibility problems with current object markers across various hazardous situations, for young-middle/aged drivers (ages 20 to 40); young-old drivers (ages 65 to 69) and old-old drivers (age 70 and older). In addition, novel object markers and pavement markings were evaluated to determine whether they improved conspicuity or understanding over the current Types 1, 2, and 3 object markers. With regard to object markers used at gore areas, the MUTCD (Section 2C.65) states that in some cases "there might not be a physical object involved [that needs to be marked], but other roadside conditions exist, such as narrow shoulders, drop-offs, gores, small islands, and abrupt changes in the roadway alignment, that might make it undesirable for a road user to leave the roadway, and therefore would create a need for a marker."
In the laboratory experiment conducted by Lerner et al., 64 subjects viewed color photographic illustrations of an object marker situated within a roadway scene. Six stimuli were used to mark gore areas: (1) a Type-1 object marker; (2) a Type-3 object marker; (3) yellow cones; (4) green cones; (5) modified French gore signs (signs used in France, consisting of two, white isosceles triangles pointing left and right on a green background, were modified to show black triangles with a cut-out base pointing left and right on a yellow background); and (6) a novel treatment displaying double modified chevron arrows (directional arrows pointing left and right, derived from the chevron alignment sign, consisting of black arrows on a yellow background). The dependent measure was the correctness of response (e.g., the subject correctly identified the hazard or described the correct driving action). Percent-correct ratings for each marker presented in a gore situation across age were as follows: Type-1 (80%); Type-3 (75%); yellow cones (46.7%); green cones (73.3%); French gore markers (56.3%); and double modified chevrons (87.5%). For drivers over age 70, the percent correct ratings were as follows: Type-1 (83.3%), Type-3 (76.8), and double modified chevron (100%). One finding of interest that should be highlighted here, is the lack of understanding of directional information presented by a solitary Type-3 object marker. In a study conducted during the problem identification stage of this research, participants were correct 39 percent of the time about the direction the marker conveyed (i.e., drive in the direction of the downward pointing stripes).
Next, the object markers were viewed on a test track by different groups of subjects in the same three age ranges to determine daytime and nighttime detection distances. Each trial began at 1,000 ft. An experimenter drove along a test track toward the object marker, with the subject seated in the passenger seat. When a subject was just able to discriminate some feature of the marking, the experimenter was told to stop, and this detection distance was recorded. The experimenter then continued to drive toward the marking, stopping every 50 ft, at which point the subject described the salient features of the marking. In addition to the markers described above, the following pavement markings were also evaluated: a double edge line pavement marking, and diagonal hash mark pavement markings. The first finding of interest was that the threshold distances were significantly greater for the post-mounted marker types than for the pavement marking types. There were no main effects of age group or marker type for the pavement markings and no interactions between these variables. The mean nighttime detection distance for the hash marks was 224 ft and for the double edge line 226 ft. By comparison, all of the subjects detected all of the other post-mounted markers at 1,000 ft at night under low-beam headlight illumination. This finding underscores the importance of including post-mounted markings at gore areas, to supplement pavement markings applied in these areas. There was no age group effect or interaction between age group and post-mounted marker type for detection distance.
In terms of nighttime symbol recognition distance, the Type-1 and Type-3 object markers had the highest mean recognition distances 790 ft and 910 ft, respectively). While Type 3 was the single best performer, both had significantly higher mean symbol shape recognition distances than all other markers. Next in terms of nighttime symbol recognition distance were the Type-2 object marker, the double modified chevrons, and the French gore signs, which were not significantly different from each other. Significant main effects of age group and marker type were found but there was no significant interaction between the two.
The only marker that resulted in a significant change in daytime detection distance was the small Type-2 marker, for which detection distance was significantly reduced for the age 70 and older group versus the young/middle-aged subjects. The mean detection distance of the Type-2 object marker was 919 ft by the young-old drivers and 803 ft by the old-old drivers, compared to 1,000 ft for the young/middle-aged drivers. In fact, all of the subjects in the age 65 to 69 age group saw all other post-mounted markers at 1,000 ft, and the drivers over age 70 saw all other post-mounted markers at distances ranging from 973 to 1,000 ft. The Type-2 marker also resulted in significantly shorter daytime color recognition distances than any other post-mounted marker type.
Finally, a limited validation study conducted by Lerner et al. on actual roads in Calvert County, Maryland, compared the Type-1 object marker and the double modified chevron post mounted markers at a gore situation. The Type-1 object marker produced a correct response rate of 50 percent, compared to 82.4 percent for the two modified chevron design, across driver age groups. However, the double modified chevron marker was better understood than the Type-1 marker at a gore location, only during the daytime.
Thus, based on the findings of the laboratory and controlled field studies conducted by Lerner et al. (1996), undelineated gores (i.e., without any object marker) were identified only 1.5 percent of the time by drivers age 75 and older, highlighting the importance of using object markers at such locations. Another finding of importance is that the Type-2 object marker is a poor choice for marking gore areas, particularly for aging drivers. The novel double modified chevron marker may be the best candidate for marking gore locations; however, more research would be required to enable a recommendation to be made for its use, based on its poorer nighttime performance compared to the Type-1 marker. Currently, the MUTCD only allows for the use of a Type-2 or a Type-3 marker for objects adjacent to the roadway (such as a gore). Based on the poor performance demonstrated by the Type-2 marker in the Lerner et al. research, a recommendation to use the Type-3 marker at freeway gore locations is made in this Handbook. It is also noted that, based on the comprehension data, a Type-1 marker may be a better candidate—for use as an experimental device—when other treatments have not proven successful.
To meet the needs of aging drivers, the point of controlling curvature on an exit ramp, as well as the curve speed advisory, must be highly conspicuous to create an appropriate expectancy of the required vehicle control actions. With this expectancy, aging drivers should be able to negotiate deceleration lane geometries meeting AASHTO or NCHRP guidelines competently. Raised curve delineation treatments are recommended in this regard; post-mounted delineators or chevrons are particularly effective at improving driver performance on sharp horizontal curves, as noted by Johnston, 1983; Jennings, 1984; Good and Baxter, 1986; Zador, Stein, Wright, and Hall, 1986, and Pietrucha, Hostetter, Staplin, and Obermeyer, 1996.
Studies dating back to the 1960's have addressed the effects of ramp design on driving performance; however, Koepke (1993) reported that the basic design criteria, and therefore design standards, used by governmental agencies to design exit and entrance ramp terminals have not changed in more than 30 years. Recommendations for selected design features for interchange ramps may be justified by both the changing characteristics of the driving population and the operating characteristics of the highway system. Age-related functional decreases in visual acuity, motion judgment, and information-processing capabilities cause increased difficulty for aging drivers entering and exiting highways. At the same time, traffic density has increased dramatically, resulting in more complex decision-making and divided-attention requirements at these sites. In a survey of 664 drivers age 65 and older, one-half of those surveyed (49 percent) reported that the length of freeway entry lanes was a highway feature that was more important to them now compared with 10 years ago (Benekohal, et al., 1992).
|Applications in Standard Reference Manuals|
|MUTCD (2009)||AASHTO Green Book (2011)||Traffic Engineering Handbook (2009)|
Fig 3B-8 through 10
|Pgs. 3-6 through 3-8, Sect. 3.2.3 Decision Sight Distance
Pg. 8-17, Paras. 1-2
Pgs. 10-76 through 10-79, Sect. on Auxiliary Lanes
Pg. 10-92, Para. 4
Pgs. 10-103 through 10-105, Sects. on Left-side entrances and exits & Traffic Control
Pgs. 10-107 through 10-127, Sects. on Speed-change lanes, Single-Lane Free-Flow Terminals, Entrances, and Single-Lane Free-Flow Terminals, Exits
|Pgs. 251-252, Sect. on Ramp Design
Pgs. 225-226, Sect. on Decision Sight Distance (DSD)
Pgs. 65-70, Sect. on Vehicle Performance
The difficulties aging drivers are likely to experience on freeway ramps, particularly acceleration lanes, are a function of changes in gap judgment ability resulting from a diminished capability to accurately and reliably integrate speed and perceived distance information for moving targets; reduced neck/trunk flexibility; and age-related deficits in attention-sharing capabilities. First, the requirement to yield to approaching traffic on the mainline requires a merging driver to assess the adequacy of gaps in traffic by turning his/her head to look over the shoulder and/or by using the sideview mirrors. In a survey of 297 adults ranging in age from 22 to 92, which was conducted to gain a greater understanding of the visual difficulties they encounter while driving, the aging participants reported greater difficulty judging both the speed of their vehicle and the speed of other vehicles, and expressed a concern over other vehicles "moving too quickly" (Kline,et al., 1992).
It has been shown that aging persons require up to twice the rate of movement to perceive that an object is approaching, and require significantly longer to perceive that a vehicle is moving closer at a constant speed, compared with younger individuals (Hills, 1975). Darzentas, McDowell, and Cooper (1980) used Hills' data in a simulation model to estimate conflict involvement for each class of subject as a function of main-road flow and speed. In the model, a conflict occurs when a poor gap acceptance decision is made by a driver, causing an oncoming vehicle to decelerate to avoid collision. The model indicated that aging drivers were involved in more conflicts than younger drivers of the same gender, and male drivers were involved in more conflicts than females in the same age class at all flows.
Other findings describing age differences in driver behavior on acceleration ramps are reported in a National Highway Traffic Safety Administration (NHTSA) study of driver age and mirror use. In this study, which measured the time required to make a 'safe/unsafe" maneuver decision in a freeway lane-change situation, old-old drivers (age 75 and older) consistently required longer response times to make a lane-change decision than a group of drivers ages 65–74, who in turn demonstrated exaggerated response times compared with a younger control group (Staplin, et al., 1996). This was a simulator study, using large screens showing dynamic videos of overtaking vehicles, in correct perspective, as the test stimuli; also, all drivers were forced to rely on their mirror information alone to make the maneuver decision in this research. The mean response time for a lane-change decision for the oldest (75 and older) driver group in this study, across a large number of trials in which the relative speed of the overtaking vehicle was varied between 10 and 25 mph (i.e., faster than the subject's own vehicle was traveling when the video was shot), changed with changes in the target distance (separation of overtaking vehicle from driver). At close separation distances (100 to 200 ft), where virtually all aging drivers quickly decided that a lane-change maneuver was unsafe, decision latency averaged approximately 2.1 s. At a 200-ft separation distance, some drivers were more willing to merge, and required longer to reach a maneuver decision, producing a mean latency of 2.5 s. At a 300-ft separation distance and above (between the overtaking vehicle and the driver wishing to change lanes), maneuver decision latency reached an asymptote at 2.95 s, as increasing percentages of subjects accepted the available gap ahead of the overtaking vehicle.
Findings from reviews of crash rates and ramp characteristics are also relevant. Lundy (1967) found that off-ramp crash rates were consistently higher than on-ramp crash rates. However, Oppenlander and Dawson (1970) reported that at urban interchanges, 68 percent of the interchange ramp crashes occurred at entrance ramps, while 32 percent occurred at exit ramps; for rural interchanges, these percentages were reversed. Similarly, Mullins and Keese (1961) reported that in urban areas, 82 percent of the interchange crashes occurred at on-ramps and 18 percent at exit ramps. Further, Lundy's (1967) study of 722 freeway ramps in California found that the crash rate was reduced for off-ramps when deceleration ramps were at least 900 ft (274 m) long (not including the length of the taper), for on-ramps when acceleration lanes were at least 800 ft) long, and for weaving sections that were at least 800 ft long. Oppenlander and Dawson (1970) also concluded that safety was improved for on-ramps, off-ramps, and weaving areas 800 ft in length or greater. Cirillo (1970) found that increasing the length of weaving areas reduced crash rates, and increasing the length of acceleration lanes reduced crash rates if merging vehicles constituted more than 6 percent of the mainline volume. Reduced crash rates from lengthening of deceleration lanes also appears to be related to the percentage of diverging traffic, with significant safety benefits beginning when 6 percent of the mainline traffic diverges (Cirillo, 1970).
The most comprehensive work to develop guidelines for freeway speed-change lanes (SCLs) was conducted in NCHRP project 3-35 by Reilly, et al. (1989), who collected data on the entry and exit processes by videotaping 35 sites in three States. An entrance model was developed, based on gap acceptance and acceleration characteristics of drivers as determined by the controlling geometry. An exit model was developed, based on the driver's behavioral response to design geometrics. The purpose of the research was to develop new criteria that would offer greater flexibility than the (then) current AASHTO (1984) guidelines, which "do not provide the designer with the ability to reflect important geometric and traffic conditions" (Reilly et al., 1989). In this research, it was reported that the AASHTO (1984) SCL design criteria were based on the acceleration and deceleration characteristics of early-model vehicles, with little regard to traffic flow characteristics or driver behavior. The design values produced by the NCHRP project entry model for SCL length were slightly lower at low freeway speeds and significantly higher at moderate to high freeway speeds when compared with the 1984 AASHTO values. The exit model values for length were significantly higher than 1984 AASHTO values for all freeway and ramp speeds. The findings of the study suggest that for certain traffic conditions, the current SCL design criteria do not provide sufficient length for proper execution of the merge or diverge process. This is of particular importance with regard to the age-related diminished capabilities documented above.
Potts, Harwood, and Pietrucha (2001) compared the design values produced by NCHRP Project 3-35 and the AASHTO values for acceleration lane length during the conduct of NCHRP Project 20-7. They concluded that there were only limited cases where NCHRP 3-35 values exceed the AASHTO values, and voiced concern with determining acceleration lane lengths based on NCHRP Project 3-35 because the design criteria are volume dependent. For these reasons, they recommended against changing Green Book policy on acceleration lane lengths, stating, "traditionally, AASHTO has been reluctant to adopt volume-dependent design criteria because a project designed in anticipation of one volume level might not meet the criteria for the volume level that actually occurs at some future time." Bared, Giering, and Warren (1999) also compared the acceleration lane length values from NCHRP Project 3-35 to AASHTO's values, presenting models to predict the safety performance of different acceleration lengths and a procedure to determine economic benefits of lengthening acceleration lanes. Bared et al. (1999) concluded that from a safety and economic perspective, minimum lengths of acceleration lanes are comparable to the minimum lengths recommended by AASHTO rather than the longer lengths recommended by NCHRP 3-35. They also concluded that higher benefit-cost ratios are possible by extending deceleration lanes compared to acceleration lanes, due to the higher crash occurrence on deceleration lanes, and that regardless of the original speed-change lane length, the benefit-cost ratio reaches a maximum for additions of 500 ft.
Another issue addressed by NCHRP 3-35 was acceleration lane geometry. Koepke (1993) reported that 34 of the 45 States responding to a survey conducted as a part of NCHRP 3-35 on SCL's use a parallel design for entrance ramps. Thirty of the agencies interviewed use a taper design for exit ramps and a parallel design for entrance ramps. The parallel design requires a reverse-curve maneuver when merging or diverging, but provides the driver with the ability to obtain a full view of following traffic using the side and rearview mirrors (Koepke, 1993). Although the taper design reduces the amount of driver steering control and fits the direct path preferred by most drivers on exit ramps, the taper design used on entrance ramps requires multitask performance, as the driver shifts between accelerating, searching for an acceptable gap, and steering along the lane. Reilly et al. (1989) pointed out that the taper design for entrance lanes poses an inherent difficulty for the driver and is associated with more frequent forced merges than the parallel design. Forced merges were defined as any merge that resulted in the braking of lagging vehicles in Lane 1, or relatively quick lane changes by lagging vehicles from Lane 1 to a lane to the left. The parallel design would thus appear to offer strong advantages in the accommodation of aging driver diminished capabilities. Indeed, aging drivers participating in focus groups voiced support for dedicated acceleration lanes (parallel entrance ramps) in place of tapered entrance ramps, and an analysis of on-ramp crashes involving aging drivers showed a decline of 1 percent in the period following replacement of tapered acceleration lanes with dedicated acceleration lanes with ramp meters (Kihl, 2005; Kihl, et al., 2004). Because the treatment in the "after" period included two countermeasures (ramp geometry plus ramp meters), it was not possible to determine singular effects of geometry and operations on crash rate reduction.
Finally, Keller (1993) provided a review of interchange design principles in need of reconsideration to accommodate aging drivers with diminished capabilities. According to this review, the factors that influence ramp alignment and superelevation design include design consistency and simplicity, the roadway user, design speed, and (stopping and decision) sight distance. Because driver reaction time is slowed when elements of ramp geometry are different than expected, design should provide for long sight distances, careful coordination between horizontal and vertical alignment, generous curve radii, and smooth coordinated transitions, particularly when complex interchange designs are unavoidable. Increasing the sight distance and simplifying interchange layout can reduce some of the effects of decreasing visual acuity, short-term memory decline, reduced decision making ability, reduced ability to judge vehicle speed, decreased muscle flexibility and pain associated with arthritis, and early fatigue and slower reaction times associated with increasing driver age. With regard to design speed, Keller (1993) stated that the ramp proper should be viewed as a transition area with a design speed equal to the speed of the higher speed terminal wherever feasible, and that few diagonal or loop ramps are long enough to accommodate more than two design speeds. Thus, the terminals and the ramp proper should be evaluated to determine the appropriate speed for design.
In terms of stopping sight distance (SSD) requirements, Keller (1993) noted that designers can reduce drivers' stress at interchanges by providing sight distances greater than the minimum SSD's. Although a brake reaction time of 2.5 s is representative of 90 percent of the drivers used in a 1971 study by Johansson and Rumar, and was used in the AASHTO SSD formula, it has been suggested that a 3.5-s perception and braking time should be used to accommodate the elderly with diminished visual, cognitive, and psychomotor capabilities (Gordon, McGee, and Hooper, 1984). Another assumption in the 1984 AASHTO calculations for SSD is a driver eye height of 3.5 ft; the eye height of aging drivers is often less. Finally, alignment affects braking distance, such that curves impose greater demands on tire friction than tangents, resulting in increased braking distance when the friction requirements of curves and braking are combined (Glennon, Neuman, and Leisch, 1985).
Keller (1993) noted that locations where SSD values do not provide the time necessary to process information and react properly highlight the importance of the use of decision sight distance (DSD). Examples of locations at interchange ramps where DSD is desirable include ramp terminals at the main road, especially at an exit terminal beyond the grade separation and at left exits; ramp terminals at the cross road; lane drops; and abrupt or unusual alignment changes. AASHTO guidelines (2004) note that sight distance along a ramp should be at least as great as the safe stopping distance. The sight distance on a freeway preceding the approach nose of an exit ramp should exceed the minimum stopping distance for the through traffic speed, desirably by 25 percent or more, although the desirable goal remains DSD.
DSD values—which include detection, recognition, decision, and response initiation and maneuver times—are provided in AASHTO (2011) Table 3-3 by design speed and type of avoidance maneuver required. Lerner, et al. (1995) measured DSD for three driver age groups (ages 20–40, ages 65–69, and age 70 and older) at six freeway lane drop locations. While perception-reaction time values measured by Lerner et al. (1995) were actually somewhat lower than the values assumed by AASHTO, they nevertheless found that the 85th percentile total time required by each age group for detection, decision and maneuvering exceeded the recommended AASHTO value of 14.5 s. The freeway total times averaged 16.5 s, 17.6 s, and 18.8 s, for the three groups (from youngest to oldest), respectively. The researchers explained that the original AASHTO work assumed free-flow traffic conditions, in which drivers were not required to wait for a gap in traffic to change lanes. The Lerner et al. (1995) study, by comparison, was conducted on heavily traveled urban freeways, and subjects often had to wait for gaps in traffic before maneuvering. This led to significantly higher maneuver times than were assumed by AASHTO. No modifications to the existing DSD standards were deemed necessary. Keller (1993), reporting on the results of a 1991 survey about distances used when locating ramp exits beyond a crest vertical curve, indicated that 15 (38 percent) of State design agencies use the safe SSD, 9 (23 percent) use the safe SSD plus 25 percent, and 12 (31 percent) use DSD.
Research has documented that: (1) freeway interchanges experience a higher crash rate than the mainline (Cirillo, 1968); and (2) urban freeway lighting has beneficial safety effects (Box, 1972). Cirillo (1968) also found a reduction in the number of interchange crashes as lighting intensity increased. Gramza, Hall, and Sampson (1980) evaluated the interchanges in the Interstate Accident Research (ISAR-2) database at which lighting had been introduced during the 10-year study period. During the daytime, there were 83 crashes before lighting and 80 crashes after lighting. At nighttime, by comparison, there were 76 crashes before lighting and 43 crashes after lighting. Taylor and McGee (1973) found a reduction in erratic maneuvers at exit lane drop sites in a before after study, when the exit area was illuminated during the "after" period of data collection.
Although nighttime driving is associated with a higher crash risk for drivers of all ages, the effects of aging on the visual system are further compounded by the effects of darkness. The aging process causes gradual declines in a variety of visual functions, including acuity, contrast sensitivity, glare recovery, and peripheral vision, making night driving especially difficult for aging drivers. Of particular difficulty is the ability to notice and recognize objects at night and in low-light conditions such as dawn and dusk, rain, fog, haze, and snow. Between age 20 and age 70, aging directly reduces contrast sensitivity by a factor of about 3.0 (Blackwell and Blackwell, 1971); aging drivers are thus at a greater relative disadvantage at lower luminance levels than younger drivers.
|Applications in Standard Reference Manuals|
|MUTCD (2009)||AASHTO Green Book (2011)||NCHRP 500 – Volume 9 (2004)||Roadway Lighting Handbook (1978)|
|Sect. 1A.13, Sign Illumination
Sects. 2E.06 & 3I.04
Sect. 4B.04, Item I
|Pg. 3-172, Para. 4||Pgs. V-21-V-22 Sect. on Strategy 3.1 B7: Improve Lighting at Intersections, Horizontal Curves, and Railroad Grade Crossings (T)||Pgs.14-15, Sects. on Complete Interchange Lighting & Partial Interchange Lighting
Pgs. 16-26, Sects. on Analytical Approach to Illumination Warrants & Informational Needs Approach to Warrants
Pgs. 42-45, Sect. on Summary of Light Sources
Pg. 71, 4th bullet
Pgs. 84-89, Sect. on Interchange Lighting
Pgs. 120-129, Sect. on Illumination Design Procedure
The impact for the aging driver of lost sensitivity under nighttime conditions should be assessed against the nature of the night driving task. Even at night, most visual information is processed by the cone or daylight system in the foveal region of the retina where fine detail is resolved. Artificial lighting raises the illumination level of the roadway environment to the photopic range so that reading and tracking functions can occur. The peripheral rod system participates primarily by alerting the driver to a weaker signal away from the foveal line of sight, which may then be oriented to by the driver with a foveal fixation. The implication of a loss in rod sensitivity is that a much brighter peripheral signal will be needed to elicit proper visual attention from the driver, and that signals now falling below threshold will be ignored. In fact, the signal may need to be 10 to as much as 100 times brighter, depending on age and object color (Staplin, Lococo, and Sim, 1990). Since both rod and cone thresholds increase with age, it is also true that more light will be needed to bring important tasks such as reading and tracking (path maintenance) above the cone limit. In a survey of 1,392 drivers ages 50 to 97, 70 percent indicated that more highway lighting is needed on freeways. These respondents identified the following areas where more lighting is needed: interchanges, construction zones, and toll plazas (Knoblauch, Nitzburg, and Seifert, 1997).
There are a number of other aspects of vision and visual attention that relate to driving. In particular, saccadic fixation, useful field of view, detection of motion in depth, and detection of angular movement have been shown to be correlated with driving performance (see Bailey and Sheedy, 1988, for a review). While these visual functions do not appear to have strong implications for highway lighting practice, it could be advantageous to provide wider angle lighting coverage to increase the total field of view of aging drivers. High-mast lighting systems can increase the field of view from 30 degrees (provided by conventional fixtures) to about 105 degrees (Hans, 1993). Such wide angles of coverage provided by high-mast lighting might have advantages for aging drivers in terms of peripheral object detection, thus easing the task of identifying ramp geometry, traffic control devices, and traffic patterns. However, while effective high-mast systems have been demonstrated (Ketvirtis and Moonah, 1995), such installations also tend to sacrifice target contrast for the increased field of view they provide.
Hans (1993) defines "high mast" as any lighting structure that rises at least 60 ft above road level. Some designs extend up to 150 ft (46 m) and higher above the ground. One pole, anchored 50 to 70 ft from the edge of the roadway, may be used to support a cluster of 3 to 12 luminaires. As a comparison, conventional cobra-head poles mounted on the shoulder support 1 or 2 luminaires, at a height of 26 to 50 ft above the road. For example, the New Jersey Roadway Design Manual defines their high-mast lighting system as one that utilizes a mounting height of 100 ft with a cluster of a maximum of eight, 400-watt, high-pressure sodium luminaires, and their conventional lighting system as one that utilizes mounting heights of 26 ft with 150-watt, high-pressure sodium luminaires for ramp application. The NJ Manual states that tower lighting (high mast) shall be considered first (over conventional lighting) for full interchange lighting, preferably using 400-watt cutoff-type luminaires; however, non-cutoff luminaires may be employed if the designer can justify their use.
The following paragraphs summarize studies that: (1) evaluated the effects of lighting on crash experience at interchanges; and (2) evaluated specific aspects of driver performance as a function of number and type of luminaires at an interchange.
Gramza et al. (1980) conducted a crash analysis of 400 nighttime crashes that occurred at 116 interchanges during the period of 1971–1976, in five States (Maine, Maryland, Minnesota, Texas, and Utah). In an analysis of the presence of high-mast lighting at interchanges, versus no lighting or other kinds of interchange lighting, the presence of high-mast lighting was found to significantly reduce total crash rates, total crashes involving fatalities and injuries, and crashes involving fatalities and injuries other than the vehicle-to-vehicle and vehicle-to-fixed-object categories (e.g., crashes caused by striking pedestrians). Table 46, taken from Gramza et al. (1980), shows the predicted effect of high-mast lighting on annual number of crashes.
|Night Traffic Volume||Urban||Nonurban|
|Non-High-Mast||High-Mast||% Decrease||Non-High-Mast||High-Mast||% Decrease|
Gramza et al. (1980) also found that although the number of lights at an interchange and the level of illumination had no significant effect on the total number of nighttime crashes, significant decreases in a variety of distinct crash types were found with increases in illumination. Increases in the illumination level—measured in lux or horizontal foot-candles (hfc)—at interchanges were associated with significant reductions in two types of crashes: vehicle-to-fixed-object crashes involving property damage, and vehicle-to-vehicle crashes involving fatalities and injuries. In addition, increases in the number of lights active at an interchange were found to significantly influence (reduce) the following two crash types: vehicle-to-fixed-object crashes involving fatalities and other injuries, and other property damage crashes. The number of lights at an interchange ranged from 0 to 114, with an average of 16 active lights and a median of 10. Thirty-two percent of the interchanges were unlit. As lighting levels increased, crash rates decreased. Illumination ranged from 0.0 lux to 10.76 lux (0.0 hfc to 1.0 hfc), with an average of 5.49 lux (0.51 hfc) for the lighted sections. These four crash types accounted for 61 percent of the crashes observed in the sample.
Since there were relatively few crashes per interchange per year, Gramza et al. (1980) employed a model to predict the number of each crash type per year, assuming three levels of traffic volume (average nighttime traffic of 5,000, 7,500, and 10,000 vehicles) at partial cloverleaf and other types of interchanges, and allowing varying levels of illumination or varying numbers of lights. The predicted relationships between traffic volume, lighting, and crash frequency showed that reductions in number of lights and in level of illumination (hfc) resulted in higher frequencies of vehicle-to-fixed-object and other property damage crashes, for all traffic volumes. Vehicle-to-vehicle crashes were also shown to increase in frequency as illumination was reduced, for all interchange types.
In addition, the findings at the level of one interchange were translated to estimate, as an overall annual impact for the five-State sample, the relative influence of the lighting variables on numbers of crashes at interchanges through three levels of night traffic volume. A level of 7.53 lux (0.7 hfc) was used to represent the allowable base of average maintained illumination. Overall, the model predicted that reductions in levels of illumination appear to cause greater increases in the number of crashes than do reductions in numbers of lights (Gramza et al., 1980).
Although the work of Gramza et al. (1980) is noteworthy in its attempt to quantify the complex relationships between interchange lighting and safety, it is critical to remember that their model was applied to data derived to fit 1975 conditions—including, by implication, both the then-current number of aging drivers and their exposure to this highway feature during nighttime operations. By contrast, present and anticipated future driving patterns of aging drivers—whose actual numbers, as well as their percentage of all drivers, will increase dramatically—show much higher use rates for freeways (Lerner and Ratté, 1991). This trend should sharply accentuate the safety impacts cited by Gramza et al.
Janoff, Freedman, and Decina (1982) conducted a study to determine the effectiveness of partial lighting of interchanges, where partial interchange lighting (PIL) was defined as lighting that consists of a few luminaires located in the general areas where entrance and exit ramps connect with the through traffic lanes of the freeway (between the gore and the end of the acceleration ramp/beginning of the deceleration ramp). A complete interchange lighting (CIL) system includes lighting on both the acceleration and deceleration areas plus the ramps through the terminus. In their survey of approximately 50 agencies which supplied information on over 14,000 interchanges and over 7,500 interchange lighting systems, it was found that 37 percent of the interchange lighting was CIL and 63 percent was PIL. An observational field study was conducted to determine the effects of lighting level (various levels of PIL, CIL, no lighting, and daylight), geometry of the interchange (straight versus curved ramps), and presence of weaving area versus no weaving area on driver behavior and traffic operations. PIL was stratified by the number of lights at each ramp, and included three levels: PIL 1 (one light), PIL 2 (two lights), and PIL 4 (four lights). CIL test sites included a full cloverleaf in suburban Baltimore, Maryland, and a three-leg interchange in suburban Philadelphia, Pennsylvania, with luminaire mounting heights of 40 and 31 ft, respectively. The dependent measures included speed and acceleration of individual vehicles traversing the interchanges; merge and diverge points of individual vehicles entering the main road or leaving it; and erratic maneuvers such as brake activations, use of high beams, and gore or shoulder encroachments.
Both field studies indicated that CIL provided a better traffic operating environment than did PIL and that any interchange lighting performed better than no lighting (although the differences were not always as great as between CIL and PIL). In particular, to the extent that traffic flow and safety are important issues, the Janoff et al. study concluded that existing CIL systems should not be reduced to PIL systems. When installing new lighting and economics are not an overriding issue, a CIL system is preferred over a PIL system. However, a PIL system with one or two luminaires per ramp will normally perform better than no lighting at far lower cost than a CIL system. PIL systems with fewer luminaires (one or two) frequently performed better than PIL systems with greater numbers of luminaires (four). This was explained by the fact that drivers may experience transitional visibility problems under the PIL conditions when they are forced to drive from dark to light to dark areas and at the same time perform complex maneuvers such as diverging, merging, and tracking a 90-degree curve.
Hostetter, Crowley, Dauber, and Seguin (1989) noted that when luminaires are not placed downstream of the physical gore of a partially lighted exit ramp, a driver proceeds from a lighted area to a nonlighted area. Citing evidence from various researchers (Boynton and Miller, 1963; Boynton, 1967; Boynton, Rinalducci, and Sternheim, 1969; Boynton, Corwin, and Sternheim, 1970; Rinalducci and Beare, 1974; and Fredericksen and Rotne, 1978), they reported that the effect of going from higher to lower levels of luminance results in a reduction in visual sensitivity, which would help explain the findings of Janoff et al. (1982) that performance under partial lighting was better with fewer luminaires.
On a final note, Bjørnskau and Fosser (1996) conducted a before-after study on a section of roadway in Norway to record driver behavior as a function of roadway lighting. One interesting finding was that the percentage of aging drivers and female drivers increased after the introduction of roadway lighting. Thus, a secondary benefit of roadway lighting (beyond its capability to reduce crashes) is increased mobility and access to goods and services for aging drivers.
It has been reported that out of 100 wrong-way crashes, 62.7 result in an injury or fatality, versus 44.2 out of 100 for all freeway or expressway crashes (Tamburri and Theobald, 1965). These data highlight the fact that wrong-way crashes are more severe than most other types. The most frequent origin of wrong-way incidents, as reported by these authors, was entering the freeway via an off-ramp.
Results of investigations of the wrong-way problem in California indicate that fatal wrong-way crashes as a percentage of all fatal crashes on freeways have decreased substantially in the last 20 years (Copelan, 1989). The actual number of wrong-way fatal crashes was the same in 1987 as it was in 1963 (about 35 per year), despite the fact that freeway travel has increased fivefold; the reduction appears to be related to the countermeasures employed by California Department of Transportation over the intervening years, including the implementation of guide and wrong-way signs and pavement markings providing better visual cues. Copelan (1989), while noting that half of the wrong-way driving on freeways was from deliberate, illegal U-turns, reported that additional improvements could still significantly reduce wrong-way crashes.
|Applications in Standard Reference Manuals|
|MUTCD (2009)||AASHTO Green Book (2011)||NCHRP 500 – Volume 9 (2004)||Roadway Lighting Handbook (1978)|
|Sects. 2A.23, 2B.37 through 2B.41
Tables 2B-1 & 2C-1
Sects. 2E.53, 3B.20, 3D.01
Figs. 3B-24, 2B-18, 2B-19
|Pgs. 10-83 through 10-87, Sect. on Wrong-Way Entry||Pg. 23, Paras. 1-2||Pgs. 373-374, Sect. on Mounting Height and Lateral Clearance|
Early studies found that the rate of wrong-way driving based on vehicle-miles of travel increased with driver age (Tamburri and Theobald, 1965). In their analysis of 1,214 wrong-way driving incidents which occurred over two 9-month periods on California highways, they found a moderate increase in incidents for drivers ages 30–39 and those ages 40–49. Over age 60, the incidents rose rapidly; and over age 70, incidents occurred at rates approximately 10 times higher than for drivers ages 16–29. Lew (1971) reported on an analysis of 168 wrong-way crashes by civilians on California freeways in which the age of the wrong-way driver was recorded. While certain age groups (i.e., 30–39, 50–59, and 60–69) were represented to an extent corresponding closely to their proportion of the driving population, other groups such as those ages 16–19, 40–49, and 70–79 deviated markedly from expectation. Drivers ages 16–19 experienced approximately one-half of the wrong-way crashes expected for their age group; drivers ages 40–49 experienced three-quarters of the rate expected; and drivers ages 70–79 experienced over twice the number of freeway wrong-way crashes than would be expected.
Aging drivers' use of signs designed to control wrong-way movements is affected by their visual performance capabilities. Letter acuity declines during adulthood (Pitts, 1982) and aging adults' loss in acuity is accentuated under conditions of low contrast, low luminance, and high visual complexity. A field investigation of the effect of driver's age on nighttime legibility of highway signs indicated that aging subjects perform substantially worse than younger subjects on a nighttime legibility task using a wide range of sign materials (Sivak, Olson, and Pastalan, 1981).
Preventative measures for reducing the frequency and severity of wrong-way maneuvers include modifications in ramp and roadway geometry, and signing and pavement markings, and the use of warning and detection devices and vehicle arresting systems. Traffic prohibition signing and marking are discussed in this section, while positive guidance signing for freeway entrances and ramp geometry and delineation were discussed under Design Element 26.
Campbell and Middlebrooks (1988), following the recommendation of Parsonson and Marks (1979) to widely separate the on- and off-ramps at partial cloverleaf interchanges, experimented with a design in which close exit and entrance ramps would be combined into one paved surface separated only by a double yellow line. Ten ramps in the Atlanta, Georgia, area were redesigned and evaluated using actual counts of wrong-way movements. Two of the ramps were monitored before and after they were converted to combined ramps. At the first location, the wrong-way rate per month before construction was 86.7; after combining the ramps, the rate fell to 0.3 per month. At the second location, the wrong-way rate was 88.6 per month. After the installation of four countermeasures (trailblazers, lowered DO NOT ENTER and WRONG WAY signs, 18-in stop bar, and 8-in yellow ceramic buttons in the centerline of the crossroad), the rate dropped to 2.0 per month. Once the ramps were combined at this second location, the wrong-way rate jumped to 30.0 per month, even when ceramic buttons, permanent signing, and pavement markings and a dotted channelizing line (i.e., pavement markings that lead turning vehicles onto the ramp) were employed.
The mixed results of the Campbell and Middlebrooks study (1988) led to the evaluation of 15 additional combined ramps in the same research project, 12 of which were partial cloverleaf, with the balance consisting of median entrance/exit ramps (designed for future access by high-occupancy vehicles to the median lanes, but during the study period were open to all traffic). The study periods ranged from 30 to 102 days. The results clearly indicated that the concept of combined exit and entrance ramps can work when signing and markings conform to MUTCD specifications. It was recommended that 8-in yellow ceramic buttons be installed along the cross street centerline if all other countermeasures do not work.
With regard to signing, Friebele, Messer, and Dudek (1971) noted that the use of oversized signs and reflectorization may be needed in locations where motorists are apt to disregard wrong-way warnings, and Copelan (1989) suggested that the larger, highly retroreflective signs may be helpful for confused or elderly drivers.
Parsonson and Marks (1979) found that lowering the DO NOT ENTER and WRONG WAY signs to 18 in above the pavement to place them in the path of the headlight beams at night and placing trailblazer signs on the on-ramp were effective, inexpensive countermeasures. Individually, these two countermeasures reduced the wrong-way incidence to about one-third to one-half of its original rate. This is consistent with California's Standard Sign Package, which specifies that the DO NOT ENTER and FREEWAY ENTRANCE packages be mounted with the bottom of the lower sign 24 in above the edge of the pavement. It also specifies that ONE WAY arrows be mounted 18 in above the pavement. The Virginia Department of Highways and Transportation (1981b) noted concern regarding the 18 in mounting height of the ONE WAY signs, however, stating that the signs may become obscured by vegetation and by guardrails (when the sign is mounted behind a guardrail). Thus, mounting height was revised for this State to 36 in, to alleviate these concerns. An additional concern with lowering the mounting height of these signs is the increased potential to impact a passenger vehicle windshield if struck by a motorist entering or exiting the freeway who strays off of the ramp and crashes into the sign support. However, wrong-way entries onto high-speed facilities can cause very serious head-on collisions in locations where there is a high incidence of wrong-way entry or a high likelihood of wrong-way entry due to geometrics. Since windshield penetration is less likely to occur at a location near the ramp terminus than at other locations because of lower travel speeds of drivers traveling in the correct direction along the ramp (who are slowing down for a stop or a signalized turn) and drivers making the wrong-way movement (who are accelerating from a turn), the anticipated benefit of increased sign conspicuity and prevention of wrong-way freeway entries is judged to significantly outweigh the risk of sign penetration.
California uses the DO NOT ENTER and WRONG WAY signs together on a single signpost, with the WRONG WAY sign mounted directly beneath the DO NOT ENTER sign (the Do Not Enter Package). This sign package is placed on both sides of the ramp. For off-ramp signing, the Standard specifies the use of at least one Do Not Enter package (DO NOT ENTER and WRONG WAY signs), to be placed to fall within the area covered by the car's headlights and visible to the driver from the decision point on each likely approach; three or four packages may be required if the off-ramp is split by a traffic island. In addition, ONE WAY arrows should be placed as close to the crossing street as possible. As they are retrofitted and newly installed, the Do Not Enter sign packages in California have high intensity sheeting (Copelan, 1989).
Increases in conspicuity distance have been reported in the literature on fluorescent signing. As stated earlier in Part I for Treatment 11, Burns and Pavelka (1995) found that signs with fluorescent red sheeting were detected by 90 percent of the participants in a field study conducted at dusk. Only 23 percent of the subjects were able to detect the standard red signs, under the same lighting conditions. To improve the daytime conspicuity of DO NOT ENTER and WRONG WAY signs, as well as conspicuity of these signs under low luminance conditions (dawn and dusk), fluorescent red sheeting is recommended. In addition, use of retroreflective sheeting that provides for high brightness at the wide observation angles typical of the sign placements and distances at which these signs are viewed (e.g., 1.0+ degrees), as well as lowering the sign heights for these signs will enhance their nighttime conspicuity under low-beam headlight illumination.
Turning to a consideration of pavement markings, Tamburri (1969) found that a white pavement arrow placed at all off-ramps pointing in the direction of the right-way movement can be effective in reducing the number of wrong-way maneuvers. However, Parsonson and Marks (1979) found that at a parclo AB loop off-ramp that has its crossroad terminal adjacent to the on-ramp, standard pavement arrows, lowered DO NOT ENTER and WRONG WAY signs, trailblazer signs, and a 24-in (600-mm) wide stop bar were not sufficient, as the ramp still showed 22.3 wrong-way movements per month. Large pavement arrows (24-ft long) and yellow ceramic buttons (with a diameter of 8 in) to form a median divider on the crossroad were required, in addition. It was specified that the ceramic buttons should touch each other to form a continuous, unbroken barrier, and should extend far enough toward the interchange structure (the freeway) to prevent a wrong-way driver from avoiding the buttons by turning early. The length required is typically 100 ft. The addition of the ceramic buttons reduced wrong-way maneuvers from a rate of 88.6 per month to a rate of 2.0 per month. Campbell and Middlebrooks (1988) also found that installing yellow ceramic buttons to the extension of the centerline of the crossroad to aid in channelizing left-turning traffic entering the freeway, in combination with countermeasures employed by the Georgia Department of Transportation as standard practice—trailblazer sign, 18-in wide stop line at the end of the off-ramp, 18-ft long arrow pavement marking, and lowered WRONG WAY and DO NOT ENTER signs—reduced wrong-way maneuvers. It was also recommended in the Parsonson and Marks (1979) study that the two-piece, 24-ft long arrow pavement marking (part of the California standard) be adopted. This use of the wrong-way arrow is described in MUTCD section 3B.20 and shown in MUTCD Figure 3B-24.
Cooner, Cothron, and Ranft (2004) provided guidance for the application of wrong-way countermeasures and treatments for freeway exit ramps. In their survey of State DOTs, they found that some agencies use red retroreflective raised pavement markers as a supplement to wrong-way pavement arrows on freeway exit ramps. TxDOT's standard wrong-way pavement arrow is comprised of raised pavement markers, arranged in a design that is slightly longer and wider than the national standard. Cooner et al. (2004) indicate that although these provide good visibility at night, they can be a maintenance concern because they are often run over, particularly on high-volume exit ramps in urban areas. This results in missing markers in wrong-way pavement arrows, or arrows that are worn in appearance. In addition to recommending the use of wrong-way pavement arrows on freeway exit ramps (comprised of retroreflective raised pavement markers in a revised design according to TxDOT's standard), Cooner et al. (2004) recommend that deficient wrong-way pavement arrows be repaired and their maintenance be made a priority. No studies demonstrating safety or driver performance benefits for this treatment were described by Cooner et al. (2004).
Description of Practice: Route shield markings on the pavement in advance of major freeway junctions are used to supplement diagrammatic or other guide signs, to provide additional confirmation to drivers that the lane leads to the desired destination. Such route shield markings are currently in use in many locations in the country including Orlando, Florida and Columbus, Ohio. Route shield markings are permitted as an option in Section 3B.20 of the 2009 MUTCD.
Anticipated Benefits to Aging Road Users: Guidance information acquired from signs by drivers who are approaching a freeway junction at high speed must be remembered as they divide attention between concurrent demands—e.g., maintaining safe separation from other traffic while negotiating lane changes and/or anticipating and reacting to other drivers changing lanes. A driver may have been exposed to route guidance information on upstream signing, but cannot recall it with confidence moments later when he/she is engaged in other tasks; and, safe operation of his/her own vehicle may preclude visual search. Under such conditions, drivers who experience age-related decline in divided attention ability and working memory should realize a disproportionate benefit with advance pavement markings that confirm which lanes lead to which destination routes, provided, as always, that these markings are applied and maintained at contrast levels sufficient to ensure legibility to an "aging design driver."
Description of Practice: According to FHWA Office of Safety (2013), numerous studies of the contributing causes and issues surrounding wrong-way driving (WWD) crashes, conducted primarily by state departments of transportation since the 1960s, indicate that WWD crashes are much more likely to result in fatalities or severe injuries than other highway crash types and highlight several factors that must be acknowledged by any WWD-related road safety audit (RSA). The NTSB's FARS analysis determined that drivers over the age of 70 are over-represented in fatal wrong-way crashes (NTSB, 2013). Categorically, there are significant human factors and environmental conditions generally associated with WWD crashes. Various research efforts have found the following correlations:
A substantial percentage of wrong way drivers are impaired by alcohol.
Over-representation of certain driver age groups, such as older drivers (particularly those over the age of 70) and younger drivers (under the age of 25).
The majority of WWD crashes that result in a fatality occur at night, when visibility of roadway attributes and signs are diminished, and a disproportionate number occur on the weekend, which potentially coincides with elevated levels of alcohol consumption amongst the driving population.
Based on this information, RSA teams should carefully consider the conditions under which to conduct an RSA that includes review of WWD crashes. The RSA should consider the potential for various human factors, such as impaired driving, older drivers with diminished eyesight, and inexperienced drivers prone to driving mistakes, to affect WWD crash potential. In addition, ATSSA (2012) has published a document containing descriptions of selected WWD treatments and case studies on their installation; that document should also be consulted when considering the implementation of WWD countermeasures.
Anticipated Benefits to Aging Road Users: Treatments such as improved lighting help drivers to better recognize the configuration of intersections and interchanges. Channelization helps to physically prohibit movements that lead to wrong-way driving where exit ramps intersect with surface streets. Signing and marking, particularly in advance of the intersection, provide the driver with important information on movements that are appropriate and reduce the likelihood of wrong-way driving.