U.S. Department of Transportation
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As Chapter 2 described, there are a number of intentional and unintentional factors that cause drivers to run red-lights. With this information, several engineering measures can now be developed that reduce the occurrence of this behavior. From an engineering perspective, red-light running may be reduced if, in general, any one of these actions is taken:
If a traffic signal is the most appropriate choice of traffic control for the intersection, it is important to ensure that the motorist can see the traffic signal far enough away from the intersection so that he/she can stop safely upon viewing the yellow and red display. Then, upon viewing the yellow, and certainly the red, ensure that signal operations and conditions do not entice the motorist to intentionally or unintentionally enter on red and ensure that a driver who tries to stop his/her vehicle can successfully do so before entering the intersection. Recognizing that there are some motorists that will intentionally violate the red signal at certain times and situations, those conditions that encourage this behavior must be minimized. Engineers should also examine whether or not the traffic signal is the most appropriate choice of control for an intersection and if it can be replaced with another form of control or design that eliminates the signal and therefore the problem.
This chapter identifies various engineering measures that can be grouped under these general solutions. For each, the measure is described, applicable design standards or guidelines in the Manual on Uniform Traffic Control Devices (MUTCD) (22) are provided, and where known, its effectiveness in reducing red-light violations and resulting crashes is presented. Other considerations in implementation and use are noted where appropriate.
Motorists who violate the red traffic signal frequently claim, "I did not see the signal." As reported in Chapter 2, 40 percent of those surveyed claim they did not see the signal and another 12 percent apparently mistook the signal indication and claimed there was a greensignal indication. While there is no doubt that many of these claims are false, there probably are situations where a more visible signal would not have been violated. For whatever reason-motorist inattention, poor vision, poor signal visibility-the motorist did not see the signal, and specifically, the red signal in time to come to a stop safely. The countermeasure for this problem is to ensure that the signal is visible from a sufficient distance upstream.
Improve Signal Visibility
Signal heads placed in accordance with the MUTCD should be visible to all motorists approaching the intersection. Although the MUTCD requires a minimum of two signal faces be provided for the major movement on an approach, locations such as that shown in Figure 3-1 are, unfortunately, not uncommon. Adherence to guidelines and standards presented in the MUTCD are needed to improve signal visibility.
The MUTCD deals with signal visibility needs in a number of ways. First, it requires (standard) that at least two signals be provided for the major traffic movement Section 4D.15). Second, although it does not require a minimum visibility distance to the signal, it does require that an advance-warning sign be used if the minimum sight distance prescribed in Table 4D-1 of the MUTCD (reproduced as Table 3-1 below) is not satisfied. Third, there is a "cone of vision" requirement that states that at least one traffic signal must be not less than 40 feet (ft.) beyond the stop line and not greater than 150 ft. from the stop line and within a 40-degree cone of vision centered on the center of the approach lanes. Finally, it provides standards for when 12-inch (in.) size signal heads are to be used instead of 8-in. heads.
|85th Percentile Speed (mph)||Minimum Sight Distance (ft.)|
|Source: Reference 22|
Placement and Number of Signal Heads
The placement (pole-mounted versus overhead) and number of signal heads have a profound effect on traffic signal visibility. Numerous studies have been conducted regarding the benefits associated with the location of the signal head and the number of heads per approach. The following is a discussion on this issue.
Signals Placed Overhead
The MUTCD does not require that signals be placed overhead rather than mounted on poles either on the roadside or in the median. Although pole-mounted signals can serve a useful purpose (as discussed later in this chapter) there are significant benefits to providing overhead-signal head displays. Overhead-signal displays help to overcome the three most significant obstacles posed by pole-mounted signal heads, which are: (1) they generally do not provide good conspicuity, (2) mounting locations may not provide a display with clear meaning and (3) motorists' line-of-sight blockage to the signal head due to other vehicles, particularly trucks, in the traffic stream. Figures 3-2 and 3-3 illustrate the line-of-sight blockage often associated with pole-mounted signals.
Studies were conducted to develop guidelines for encouraging uniformity in the design of traffic-signal configurations and improving the performance of signals as critical traffic-control devices (23). Key to this effort were recommendations regarding traffic-signal design configurations, which included the recognition of visibility obstacles. An important finding of this study was "that, in most cases, over-the-roadway signals would be required to ensure adequate signal visibility." Further studies have shown significant reduction in accidents attributed to replacement of pole-mounted signal heads with overhead-signal heads. For example, in Iowa, the safety impacts of replacing pedestalmounted signals with mast-arm-mounted signals at 33 intersections, resulted in a 32 percent reduction in total crashes (24). In Kansas City, MO, replacement of postmounted signals with mast-mounted signals at six intersections contributed to a 63 percent reduction in the number of right-angle accidents and a 25 percent reduction in the total number of collisions (25). Replacement of pole-mounted traffic signals with overhead signals is likely to result in a decrease in total crashes.
In some areas, there has been a conscious decision not to use any overhead signals for aesthetic reasons. Where this is the case, this recommendation could not be implemented, but the engineer should then consider the other measures discussed in this chapter.
Signal for Each Approach Lane
Even though the benefits of overhead signals are recognized, the number and placement of the signal heads are crucial to meeting the motorist's visibility needs. Currently, Section 4D.15 of the MUTCD only requires that "a minimum of two signal faces shall be provided for the major movement on the approach, even if the major movement is a turning movement." Under this standard, it would be acceptable to have only two signals on an approach with three or more through lanes. In such a scenario, the signals would not be placed over the center of each lane but at an equal distance, splitting the width of the three lanes. However, when a signal is positioned such that it is over the middle of the lane, it is in the center of the motorist's cone of vision, thereby increasing its visibility. The additional signal head further increases the likelihood that a motorist will see the signal display for the approach. Figure 3-4 shows a three-lane approach intersection with the required two-signal minimum. Figure 3-5 shows the same three-lane approach with a signal placed over each through lane. Figure 3-6 shows the use of two overhead signal displays for a one-lane approach.
In Winton-Salem, NC, an additional signal head was installed on one or more approaches at 11 different locations. At six locations, the additional signal head was mounted directly over a travel lane replacing a display where two signal heads served three lanes. At four locations, the additional head was mounted on the left side of the road in an effort to make the signal visible earlier to traffic rounding a right curve. At one intersection, the additional head was mounted high on a utility pole to make the signal visible above a vertical curve. For all intersections combined, right-angle crashes caused by motorists on the intersection approaches where the auxiliary heads were installed declined by more than 46 percent combined. Total crashes decreased significantly at five of the 11 intersections (26). Care should be taken in reporting the magnitude of these findings since the study was a simple before-and-after study that did not account for other factors such as regression-to-mean that could have contributed to the crash reduction.
In British Columbia, the Insurance Corporation of British Columbia (ICBC) has adopted a specific design concept to combat the traffic signal visibility problems. The ICBC co-sponsored, with municipalities and the provincial road authority, the installation of additional primary overhead signals per lane, as well as the exploration into installing pole-mounted secondary (left side) and even tertiary (right side) signal heads at signalized intersections throughout the province (see Figure 3-7). The ICBC researched this topic thoroughly and concluded that there is a significant safety benefit to this type of signal-head installation (27). This research supports similar findings of an early study (23), which concluded "mixed configurations (combining overhead and post-mounted heads) are generally better than either all-post or multiple overhead configurations, except that the box span performs as well as the mixed configurations."
Based on these findings, the preferred signal configuration to improve road user visibility needs is to provide an overhead-signal display, centered over the middle of each approach lane (two overhead signals for a one-lane approach) and, if necessary for reasons cited in this report, supplemental post-mounted signals. Figure 3-8 provides a photograph of this signal configuration. For very wide approaches (perhaps four or more lanes) it may not be practical to provide an overhead signal for each lane. Therefore, for very wide approaches, placement of overhead signals directly over each lane line of the approach may be sufficient to address the overhead visibility needs.
Traffic signals should be visible from a minimum distance as prescribed by Table 4D-1 of the MUTCD (previously reproduced as Table 3-1 on page 18) and be horizontally placed at the intersection a maximum of 150 ft. from the stop line (if a 12-in. signal lens is used) as shown in MUTCD Figure 4D-2.
There are situations where these design criteria cannot be met. The approach to the intersection may be on a curve, which restricts the sight distance, making it impossible to meet the visibility distance criteria without drastic changes to the roadway. Additionally, at wide intersections the signals may have to be placed beyond the 150-ft. limit. Where this is the case a supplemental signal should be provided on the nearside approach. Figure 3-9 shows how this was accomplished at an intersection in Vienna, VA. As illustrated in the figure, the intersection approach is curved. Without the supplemental signal, the drivers would not be able to see the upcoming signal due to the horizontal curvature.
Similar to the treatment in Vienna, a supplemental-pole signal, even using a double red signal, was provided for an intersection in Naperville, IL where the signalized intersection is in the middle of a reverse curve. In addition to the supplemental signal, a BE PREPARED TO STOP WHEN FLASHING sign and flasher were used. Prior to the installation of the supplemental signal and advanced-warning devices, there was an average of three severe crashes and one fatal crash per year. After installation in 1996, there only has been one severe crash and no fatal crashes to this date (28). The supplemental signal, which has a double red display, is shown in Figure 3-10.
Increasing the size of the display improves signal visibility. As specified in the MUTCD, there are two nominal diameter sizes for vehicular signal lenses, 8 in. and 12 in. Combinations of these sizes can be used in a single signal head, although an 8-in. signal lens for a circular red signal cannot be used in combination with a 12-in. signal lens for a circular green signal indication or a circular yellow signal indication. Obviously, a signal lens that is 50 percent larger than the minimum 8-in. lens will be visible from a longer distance. Figure 3-11 provides a visual comparison of these different sized traffic signals.
The MUTCD stipulates that 12-in. signal lens (standard) shall be used under the following conditions:
Furthermore, it is recommended (guidance) in the MUTCD, that the 12-in. signal lens (for all signal indications) be used for the following conditions:
Even if none of these two sets of conditions exist, using 12-in. signal lenses should be considered for all signals, and especially those displaying red indications, to increase signal visibility.
Some implementations have considered the impact of 12-in. signal lenses on crash occurrence. Under the Systematic Safety Improvement Program, the City of Winston-Salem, NC has identified, treated and evaluated countermeasures at crash locations since 1986. One such improvement was to replace existing 8- in. lenses with 12-in. lenses on at least one approach at 55 locations throughout the city. Using a simple beforeand- after study, Winston-Salem reported a 47 percent decline in right-angle crashes caused by motorists on the upgraded approach at all treated locations combined (26).
Winston-Salem is not the only city to use 12-in. signal lenses. In fact, the policy for the City of Troy, MI now requires a 12-in. lens for all signals (red, yellow, green) leaving no 8-in. lenses in Troy (29). Similarly, Naperville, IL has a policy to use 12-in. lenses to ensure that the signal indication can be well seen. Additionally, the British Columbia Ministry of Transportation and Highways has adopted 12-in. (300-mm) signal lenses for the red, yellow and green as the new provincial standard for overhead (primary) signal heads (30).
The line of sight between the signal display and the point of required visibility is critical to the motorist's ability to see the signal head. One study (23) researched the importance of line of sight to signal visibility. Some of the key findings were:
"An analysis of human factors principles affecting the design of traffic-signal configurations revealed that the driver's perception-response tasks depend on his position on the approach. A conceptual model of these tasks was developed that identified and defined three distinct zones on the approach. Important aspects of signal configurations included placing signal indications as close to the line of sight as possible and, also, placing at least one signal head in a consistent location known to and predictable by the driver.
Optical aspects of traffic-control signals were also investigated. The major variables affecting signal configuration design were found to be the distance at which the signal first becomes visible and the offset of the signal position from the line of sight. A comparison of required signal illumination at the driver's eye and luminance characteristics of commercially available traffic signals showed that, in most cases, over-the-roadway signals would be required to ensure adequate signal visibility. This comparison also led to development of specific rules for the use of oversized signal indications."
Therefore, the signal head should be installed as close as practical to the projection of the driver's line of sight. Care must be taken to eliminate obstacles, which block the motorist's line of sight, such as utility cables/wires, structures, vegetation, or large vehicles in the traffic stream. In addition, the following are other measures that can be applied to enhance the motorist's line of sight for improved signal visibility.
The optically programmed or visibility-limited signals limit the field of view of a signal. This is similar to the purpose of louvers; however, visibility-limited signals allow greater definition and accuracy of the field of view. For example, programmable lens are used to control the motorist's lateral or longitudinal field of view. Lateral separation is useful for instances such as separating left- or right-turn lanes or locations with adjacent parallel roadways like a frontage road. An example of longitudinal separation (also known as distance separation) is for closely spaced intersections. Additionally, visibility-limited signals do not reduce the intensity of the visible light and also do not have the problem of snow and ice build-up or bird nests as sometimes incurred with louvers and visors.
The MUTCD speaks of visibility-limited signals mostly with regard to left-turning traffic at an intersection. Two examples are presented below:
Additionally, the MUTCD permits the use of visibilitylimited signal faces in situations where the road user could be misdirected, particularly when the road user sees the signal indications intended for other approaches before seeing the signal indications for their own approach.
There are a few concerns and extra precautions necessary when working with visibility-limited signals. Because the field of view is restricted and requires specific alignment, these signals require rigid mounting instead of suspension on overhead wires. Additionally, there is some concern associated with glare and the limitations of seeing the signal. These signals have also been known to create driver confusion in a few specific instances. In these instances, the signal initially appears like there is no indication-a malfunction. However, as the driver gets closer to the intersection and even passes through, they notice the signal does indeed show an indication. At that point it may be too late to stop as the vehicle is already in the middle of the intersection, waiting to make a left turn. Signal-visibility alignment requires attention both in design and in field maintenance.
The addition of a visor to a traffic-signal head that is in direct sunlight can improve visibility of the signal by providing additional contrast between the lens and the signal head. There are different types of visors including complete circle (or tunnel), partial (or cutaway) and angle visors. Cut-away visors are preferred as snow and water cannot accumulate at the bottom of the signal indications. Additionally, cut-away visors reduce the problem of birds nesting in the visor.
The MUTCD requires that "in cases where irregular street design necessitates placing signal faces for different street approaches with a comparatively small angle between their respective signal lenses, each signal lens shall, to the extent practical, be shielded or directed by signal visors, signal louvers, or other means so that an approaching road user can see only the signal lens(es) controlling the movements on the road user's approach." Additionally, the inside of signal visors should have a dull black finish to minimize light reflection. The MUTCD also recommends using signal visors, which direct the light without reducing the intensity of the light, in lieu of signal louvers.
Louvers are used to avoid confusion on intersection approaches where approaching motorists may be able to see the signal indication for another approach, typically due to a skewed approach angle at the intersection. The purpose of a louver is to block the view of the signal from another approach. They are similar to angle visors but are better in limiting signal visibility to a narrow cone to the front of the signal. The problem with louvers is that they reduce the amount of light emitted from the signal and require higher luminance to obtain the same visibility as a signal without a louver.
As stated in the discussion of visors, louvers may be used to limit the view of the signal by approaching motorists at intersections with a small angle between signal lenses. However, it is stated in the MUTCD that signal visors should be considered as an alternative to signal louvers because of the reduction in light emitted caused by the louvers. The MUTCD requires the entire surface of louvers have a dull black finish to minimize light reflection and to increase contrast between the signal indication and its background. Figure 3-12 shows louvers on traffic-signal heads at an intersection with closely spaced approaches.
Improve Signal Conspicuity
In addition to improving the visibility of a traffic signal, various countermeasures can be applied to capture the motorist's attention, i.e. to make the signal more conspicuous. The following are measures, some of which are found in the MUTCD, that should be considered in improving the signal conspicuity.
Providing two red-signal displays within each signal head should increase the conspicuity of the red display and further increase the likelihood that the driver will see the signal. While "doubling-up" on the red signal section is not normally needed, where there is a high incidence of red-light running, the engineer may want to consider this option. It is permitted by the MUTCD to repeat a signal indication within the same signal face (section 4D.18). The proper alternative arrangements of two red-signal sections are illustrated in Figure 3-13, excerpted from the MUTCD. A photograph of this measure is shown in Figure 3-14.
An evaluation of this treatment applied at nine locations in Winston-Salem, NC showed a 33 percent decrease in right-angle crashes caused by motorists on the upgraded approaches following implementation (26).
An LED traffic signal module is made up of a lens and an array of individual LEDs that are tiny, purely electronic lights that display a single color. Each LED is about the size of a pencil eraser. LEDs can be used to replace an incandescent lamp and colored lens that make up a traditional traffic-signal optical unit.
LED units are used for three main reasons: they are very energy efficient, are brighter than incandescent bulbs and have a longer life increasing the replacement interval (31). For example, in producing the same amount of light as a traffic signal with incandescent lamps, a LED traffic signal uses 90 percent less power. LEDs also emit light that is brighter because the LEDs fill the entire surface of the traffic bulb and also provide equal brightness across the entire surface. Literature providing data that would substantiate that LED signals are brighter compared to incandescent bulbs could not be found. However, it was observed by the author that at two intersections on an arterial in South Carolina, where one of the two signal displays was replaced with a red LED signal, that the LED signals were noticeably brighter and more conspicuous than the adjacent signal with the incandescent bulb. Finally, LED traffic signal modules have service lives of 6 to 10 years as compared to incandescent bulbs that have a life expectancy of only 12 to 15 months.
Section 4D.18 of the MUTCD supports the use of LED traffic-signal modules as a traffic-signal optical unit. The MUTCD states that traffic signals should conform to ITE standards (32) with regard to the intensity and distribution of light for a signal indication. At this time, only the red and green lamps meet specifications for traffic applications. Arrows and yellow LEDs that meet specifications are not currently available.
As stated previously, one of the major benefits of LED traffic-signal modules is that LEDs are brighter, which is especially helpful during poor weather or bright sunlight. However, there may be the potential for a glare problem at night because of the brightness.
The light output of LED traffic-signal modules is more directional than the output for traffic-signal optical units with incandescent lamps. As a result, signal indication visibility for some installations is limited to a narrow cone of vision below the horizontal axis of the signal face. If signal heads with LED traffic-signal modules are used, they should be mounted on mast arms. If they are installed on a span wire they are vulnerable to wind that can make the signal head tilt backwards and forwards, making the signal appear to be in flash mode. This is commonly referred to as "blanking." Even if they are mounted on mast arms, there is the possibility that the signal indication may not be visible due to the approach grade to the intersection. "Blanking" can be avoided by tethering the signal to a pole. If tethering is not a viable solution or if reduced visibility is caused by the approach grade, the LED application can be modified to an expanded pattern that increases the vertical visual cone of the LED by increasing the LED count and modifying the lenses (33).
Laboratory research has found that with brighter lights, there are quicker reaction times and fewer missed signals among test subjects (34). Although it was difficult to find a field study that confirmed the effect of LEDs on intersection safety as measured by signal violations, many cities are installing LEDs. For example, in 1998, the City of Scottsdale, AZ initiated a program to convert all of the city traffic signal's red and green indications to LEDs. The city stated four ways that LEDs improve safety at signalized intersections, including a reduction in signal indication outages, elimination of "phantom illumination" caused by colored lenses, longer re-lamping cycle (which reduces the time traffic is disrupted due to maintenance) and the ability to operate on battery backup systems during power outages (35). Bonneson's research (39) reveals that the use of yellow LEDs may reduce red-light running by 13 percent.
Backplates, as shown in Figure 3-15, are commonly used to improve the signal visibility by providing a black background around the signals, thereby enhancing the contrast. They are particularly useful for signals oriented in an east-west direction to counteract the glare effect of the rising and setting sun or areas of visually complex backgrounds. Guidance for their use for target value enhancement against a bright sky or bright or confusing background is provided in the MUTCD (Section 4D.17) for these very conditions. The MUTCD (Section 4D.18) requires the front surface of the backplate to have a dull black finish "to minimize light reflection and to increase contrast between the signal indication and its background." In many jurisdictions, it is general practice to use backplates for all signal heads, not just those in the east-west direction.
At six locations of varying types in Winston-Salem, NC backplates were added to signal displays on one or more approaches to call attention to the signal display. Angle crashes caused by motorists on the approach where the backplates were installed declined by more than 31 percent at all locations combined (26).
In British Columbia, Canada, an evaluation was conducted of high-intensity yellow retroreflective tape on the backplates of signals at six intersections on an arterial in Saanich (36). The authors hypothesized that the framed signal heads would be more visible to motorists at night and the safety of the intersection would improve. Figure 3-16 shows a daytime photograph of a signal head with the high intensity tape around the backboard.
Acomparison of crash frequency for a three-year period after installation, compared to one year before, showed that the number of night crashes stayed the same the first year (14 crashes) but decreased significantly (five and three crashes) in the subsequent two years. Volume levels actually increased in each of the four years (36). The use of retroreflective tape on the backplate is contrary to the MUTCD standard requiring a dull black finish. Hence, its use in the United States would require experimentation approval from FHWA.
One precaution to note when using backplates is the additional load on the mast arm or cable caused by the backplate. This is due to the additional weight and added wind load. The additional load must be incorporated into the design of the signal-head mast arm and/or other signal parts.
Strobe lights have been used in rare occasions as a supplement to red signals to attract the attention of the motorist and provide emphasis on the signal. A strobe light, which flashes within the red signal display, can have either of two shapes-halo or horizontal-and be either of two mechanisms-xenon tube or light emitting diode (LED). Typically, a strobe light will have a flash rate of once per second. They can be used with both incandescent and LED signals. One state's guidelines (37) for the use of strobe lights are as follows:
There has not been a comprehensive study of this device, and the present limited evaluation has shown mixed results. A 1994 study in Virginia of six intersections with one and two strobe lights had both increases and decreases in rear-end and angle accidents-accident types that should be affected by this measure (38). The overall conclusion of the researcher was that there is no clear benefit from using strobe lights and that other measures, cited in this report, should be tried.
While mentioned as a possible measure, care should be taken in using this device. Because conclusive evidence has not shown a reduction in crashes, FHWA's current position is that they will no longer be approved for experimentation. Since this device is not approved by FHWAand not included within the MUTCD, an agency using this device may be subject to liability in the event of litigation resulting from a crash.
Recalling the general solutions to red-light running presented in the introduction to this chapter, the second solution is to increase the likelihood of stopping for the red signal, once seen. Intersections and intersection devices should be carefully engineered so that the motorist is not enticed to intentionally enter the intersection on red. This may include providing additional information to the motorist regarding the traffic signal. With the additional information, the probability that a driver will stop for a red signal may increase. Additionally, the intersections must be designed in such a way that a driver who tries to stop his/her vehicle can successfully do so before entering the intersection on red. For example, an improvement in intersection pavement condition may increase the likelihood of stopping by making it easier for the driver to stop. These types of countermeasures deal with the following reported causes of red-light running:
Increase Likelihood of Stopping Through:
Most of the countermeasures discussed in this section are not innovative or required intersection elements, but rather are treatments used occasionally for specific reasons at targeted locations. Installation of these countermeasures requires a careful evaluation of the location and use of engineering judgment. The evaluations of specific implementations discussed below provide useful information when addressing the solution to a specific location.
When the primary traffic-control device used is a traffic signal, the appropriate sign is the SIGNAL AHEAD sign (W3-3) shown in Figure 3-18.
The MUTCD requires an advance traffic control warning sign when "the primary traffic-control device is not visible from a sufficient distance to permit the road user to respond to the device." For a traffic signal, the visibility criterion is based on having a continuous view of at least two signal faces for a distance specified in Table 4D-1 of the MUTCD (See Table 3-1). The MUTCD also permits the use of this device even when the visibility distance is satisfactory. In addition, the MUTCD allows for the use of a warning beacon with the sign typically flashing yellow lights on either side or on top and bottom of the sign. The placement of this sign prior to the intersection is a function of the approach speed. Table 2C-4 in the MUTCD provides the recommended distances.
The purpose of an advance-warning flasher (AWF) is to forewarn the driver when a traffic signal on his/her approach is about to change to the yellow and then the red phase. In North America, there are three general types of advanced warning devices and the decision of which to use is based on engineering judgment. These AWFs include:
Examples of different field implementation of the signs are shown in Figure 3-19 and Figure 3-20.
The effectiveness of AWFs is measured by vehicle speeds approaching the intersection, the number of redlight violations and its effect on accidents. A beforeafter study was conducted at one intersection in Bloomington, MN (40). At the intersection, BE PREPARED TO STOP and WHEN FLASHING signs were pedestal mounted and accompanied by dual 8-in. yellow beacons. Data were collected immediately after installation of the AWFs and again one year after installation. During the before-period, the yellow interval was 6 sec. and the all-red interval was 2 sec. After the installation of the AWFs, the all-red was reduced to 1 sec. Data collected included the number of red-light violations and speeds, vehicle type (car versus truck), time after the onset of the red interval when the violation occurred and time of day of violation.
From these data parameters, the authors concluded the AWFs were effective in reducing the number of overall red-light violators, the number of trucks violating the red-light and the speed of the violating trucks. One year after installation, there was still a reduction in overall, car and truck red-light violations, as well as a slight decrease in the average speed of violating trucks. However, this was an increase from the previous years after-data indicating that the effectiveness had decreased over time. Additionally, the study did not employ a control or comparison group of intersections. Therefore, the changes observed could have been due to something other than the AWFs (for example, regression-to-mean).
Another study utilized and analyzed data from British Columbia using two different methods (41). Models were used to develop expected accident rates at 106 signalized intersections for total, severe and rear-end accidents. Twenty-five of these intersections had AWFs. Although the results indicate that intersections with AWFs have a lower frequency of accidents, the difference between those with AWFs and those without is not statistically significant. An additional before-andafter study was performed for the 25 intersections equipped with AWFs to estimate the accident reduction specific to each location and its approach volumes. A correlation was found between the magnitude of the minor approach traffic volumes and the accident reduction capacity of AWFs, showing that AWF benefits exist at locations with moderate to high minor approach traffic volumes (minor street AADT of 13,000 or greater).
Another warning device that has been used to alert drivers to the presence of a signal are transverse rumble strips. Rumble strips are a series of intermittent, narrow, transverse areas of rough-textured, slightly raised, or depressed road surface (22). The rumble strips provide an audible and a vibro-tactile warning to the driver. When coupled with the SIGNAL AHEAD warning sign and also the pavement marking word message- SIGNAL AHEAD-the rumble strips can be effective in alerting drivers of a signal with limited sight distance. This treatment is illustrated in Figure 3-21.
There are no known studies reporting on how this treatment can reduce red-light violations or the resulting crashes; hence their use should be restricted to special situations. If used, they should be limited to lower-speed facilities (less than 40 mph) and be reserved for locations where other treatments have not been effective.
When a motorist wanting to turn left approaches a signalized intersection using left-turn protected only mode, he/she may be confused with the combination of two or more signals displaying a green ball for the through movement and a left-turn signal displaying a red ball or red arrow. To compensate for this, a sign that clearly identifies the left-turn signal is to be used. The LEFT TURN SIGNAL sign provides additional information not given in the actual signal indication to the driver by specifying the control device for different intersection movements. This is illustrated in Figure 3-22. Such information may eliminate driver confusion when approaching an intersection and prevent red-light running for left-turning traffic.
The MUTCD provides information regarding the use of the sign for different modes controlling the left-turns (protected, permissive, protected/permissive, variable left-turn) and signal arrangements for an approach (shared versus separate). For instance, (See MUTCD Section 4D.06.C1) under protected/permissive left-turn phasing and separate signals for the left-turn and through movements that do not display the same circular signal indications, a LEFT TURN SIGNAL sign (R10-11) and a LEFT TURN YIELD ON GREEN (R10-12) sign is required. For protected left-turn phasing where the left-turn signal includes a circular red, left-turn yellow arrow and a left-turn green arrow, the circular red must not be seen by the through traffic or the signal must clearly be designated as the left-turn signal. The circular red can be limited by using hoods, shields, louvers, positioning, or design. Alternatively, a LEFT TURN SIGNAL sign can be used
According to NHTSA, 2,627 fatal crashes and 215,000 injury crashes in the year 2000 occurred during rainy weather conditions. This is approximately 7 percent of all fatal crashes and 10.4 percent of injury crashes occurring on wet pavement. Additionally, another 2.4 percent of fatal crashes and 3 percent of injury crashes occurred during snowy or sleeting weather conditions, likely on wet pavement (42).
As a vehicle approaches a signalized intersection and slows to stop for a red-light, it may be unable to stop due to poor pavement friction and as a result, proceed into the intersection. A vehicle will skid during braking and maneuvering when frictional demand exceeds the friction force that can be developed at the tire-road interface. The friction force is greatly reduced by a wet pavement surface. A water film thickness of 0.05 mm reduces the tire pavement friction by 20 to 30 percent of the dry surface friction. Therefore, countermeasures to improve the pavement condition should seek to increase the friction force at the tire-road interface and also reduce water on the pavement surface (43).
The coefficient of friction is most influenced by speed; however, many additional factors affect skid resistance. This includes the age of the pavement, pavement condition, traffic volume, road surface type and texture, aggregates and mix characteristics, tire conditions and presence of surface water. Countermeasures to improve skid resistance include asphalt mixture (type and gradation of aggregate as well as asphalt content), pavement overlays and pavement grooving. Additionally, countermeasures such as SLIPPERY WHEN WET signs and reducing the speed limit can also be used. The MUTCD permits a SLIPPERY WHEN WET sign to be used to warn of a possible slippery condition. The sign is to be placed an appropriate distance prior to the condition and at appropriate intervals along the affected section.
The third general solution presented at the beginning of this chapter is to remove the reasons for intentional violations. The countermeasures presented in this section are mainly intended for those violators who "push the limits" of the signal phasing or try to beat the yellow signal. Previous surveys indicate that the common reasons drivers speed up and try to beat a yellow light include being in a rush and saving time. Although these drivers may not have intended to violate the red signal, they did intentionally enter towards the end of the phase knowing that there was the potential that they would violate the signal. Often times, these drivers do miss the yellow and end up running the red. The countermeasures presented in this section all relate to signal timings.
Address Intentional Violations Through:
Installing the optimum signal timings is important to ensure respect for traffic signals. The MUTCD recommends signal timings be reviewed and updated on a regular basis (every 2 years) to ensure that it satisfies current traffic demands. There are many different and specific signal countermeasures that can be implemented regarding signal timing. The range in countermeasures includes changes to the signal system (such as progression) as well as changes to the signalcycle length and individual signal phases (such as the yellow interval). Some of these countermeasures are discussed in the following sections beginning with system level changes and narrowing to changes in specific signal phases.
Poor signal timings are not only inefficient, but may cause a driver to become frustrated and respond inappropriately to the signal. The traffic demands at each intersection must be carefully accounted for when the phase sequence and timings are developed. Once these timings are developed, the relationship of the signal to other signals must be considered.
Interconnected signal systems provide coordination between adjacent signals and are proven to reduce stops, reduce delays, decrease accidents, increase average travel speeds and decrease emissions. An efficient signal system is also one of the most costeffective methods for increasing the capacity of a road. With reduced stops, the opportunity to run red-lights is also reduced. In addition, if drivers are given the best signal coordination practical, they may not be as compelled to beat or run a red signal.
Proper timing of signal-cycle lengths can reduce driver frustration that might result from unjustified short or long cycle lengths. Timing of the various signal phases is based on the characteristics of the intersection and the individual approaches. Signal timing includes the green, yellow and red phase for each approach as well as the overall signal-cycle length.
Although there are federal and state standards that bound signal timing, there are also local or regional practices of signal timing. There are philosophies and considerations that support both shorter and longer cycle lengths for reducing signal violations. The effects of cycle length vary on traffic and driver. Drivers and traffic engineers may perceive shorter cycle lengths as more efficient as vehicles have shorter periods where they have to remain stopped. A driver that knows the wait is not excessive may be less inclined to beat the yellow or run the red. Conversely, under higher traffic volumes, the short cycle length may not be sufficient to clear all queues and drivers may find themselves waiting through two or more cycles. This may cause an increase in driver anxiety resulting in an increase in drivers attempting to beat the yellow and violate the red signal. With longer cycle lengths, drivers strive to get through the signalized intersection or suffer the perceived long delay associated with sitting for the red signal. However, many traffic engineers use longer cycle lengths to move significant volumes on the mainline of arterial roadways. By providing a sustainable progression along a corridor, the saturated roadway can move higher volumes and reduce queue lengths. Delays associated with numerous start-up times are also diminished if progression is maintained.
When comparing cycle lengths, it should be noted that with longer cycle lengths, there are actually fewer numbers of times per hour when drivers are confronted with the yellow and red signal intervals. For example, when comparing a cycle length of 1 min. to 2 min., in an hours time in the 1-min. cycle, there will be twice as many opportunities for drivers to be confronted with the changing signal from green to red. Consequently, the longer cycle length does reduce the number of opportunities for traffic-signal violations.
After consideration of the pros and cons, one of the best tools to utilize in determining signal-cycle length is computer simulation and optimization. The computer generated optimized cycle length combined with the traffic engineers' knowledge and experience should result in the most efficient traffic-signal timing practical. As part of signal-timing strategies, the need to address specific times of day should be included. For example, typical timing plans would include multiple plans to accommodate the morning or afternoon peak periods, midday, late night, weekends, etc.
The MUTCD (22) requires that a yellow-signal indication be displayed immediately following every circular green or green-arrow signal indication. It is used to warn vehicle traffic that the green-signal indication is being terminated and that a red indication will be exhibited immediately thereafter.
A properly timed yellow interval is essential to reduce signal violations. An improperly timed yellow interval may cause vehicles to violate the signal. If the yellow interval is not long enough for the conditions at the intersection, the motorist may violate the signal. Motorists have some expectancy of what the yellow interval should be and base their decisions to proceed or stop based on their past experiences. In order to reduce signal violations, the engineer should ensure that the yellow interval is adequate for the conditions at the intersection and the expectations of the motorists.
In many jurisdictions, the yellow-change interval is followed by an all-red interval. During this all-red "clearance' interval, the red-signal indication is displayed to all traffic. The yellow interval and all-red interval are often referred to collectively as the change period or change interval. The all-red interval is addressed separately in a subsequent section.
There is currently no nationally recognized recommended practice for determining the change interval length, although numerous publications provide guidance including the MUTCD (22), Traffic Engineering Handbook (44), and the Manual of Traffic Signal Design (45). The MUTCD provides guidance that a yellow-change interval should have a duration of approximately 3 to 6 sec., with the longer intervals reserved for use on approaches with higher speeds.
In the current edition of ITE's Traffic Engineering Handbook (44), a standard kinematic equation is provided as a method to calculate the change interval length. The equation for calculating the change period, CP, is as follows:
The principal factors that are taken into account in the development of the change period are:
The equation allows time for the motorist to see the yellow signal indication and decide whether to stop or to enter the intersection. This time is the motorist's perception-reaction time, generally 1 sec. It then provides time for motorists further away from the signal to decelerate comfortably and motorists closer to the signal to continue through to the far side of the intersection. These times are dependent on the characteristics of the traffic and the roadway environment. If there is a grade on the approach to the intersection, the equation adjusts the time needed for the vehicle to decelerate.
If available, the 85th percentile speed should be used as the approach speed in this equation. In the absence of 85th percentile speed, some jurisdictions use posted speed as the approach speed. In most cases, using the 85th percentile speed will produce intervals that are more conservative (i.e., longer). In no case should the approach speed used in the calculation be less than the posted speed limit.
The deceleration rate of 10 ft./sec.2 suggested by ITE is based on a comfortable deceleration rate that has been supported by research. The 2001 American Association of State Highway and Transportation Official's A Policy on Geometric Design of Highways and Streets, otherwise known as the "Green Book," (46) recommends 11.2 ft./sec.2 for determining stoppingsight distance. They note that this is a comfortable deceleration for most drivers. The deceleration rate suggested by ITE is a more conservative deceleration rate for purposes of calculating the yellow interval and will result in longer intervals.
Various studies have evaluated the relationship between the length of the change interval and the occurrence of signal violations. Retting and Green (47) examined redsignal violations in New York where the yellow or allred intervals were shorter than a practice proposed by ITE in 1985 (48) that is similar in calculation to Equation 1. They conducted a before-and-after study with a control group at 20 approaches. For the afterperiod, the researchers retimed the yellow interval at four sites, the all-red interval at five sites, and both the all-red and the yellow at four sites. Seven sites were used as the control group. The yellow retiming increased the yellow change interval by 0.5 to 1.6 sec., depending on the intersection. The all-red retiming increased the red-clearance interval by 0.8 to 3.6 sec. The researchers recorded the number of cycles with red-signal violations and the number of cycles with late exits at the intersections. Red-signal violation cycles were defined as cycles where at least one of the vehicles on the approach entered the intersection on red. Lateexit cycles were defined as cycles where at least one vehicle from the approach was still in the intersection at the release of conflicting traffic.
Logistic regression was used to analyze the data. The researchers concluded that increasing the length of the yellow signal towards the ITE proposed practice significantly decreased the chance of red-signal violations. They also found that late exits decreased as the all-red interval increased. It appeared that sites with shorter yellow signals had more late exits. Increasing the yellow to ITE-suggested values was as effective as increasing the all-red clearance interval at decreasing the occurrence of late exits.
Wortman et al. (49) conducted a similar before-andafter study at two intersections in Arizona. In the afterperiod, the yellow interval was extended from 3 sec. to 4 sec. The researchers observed a statistically significant reduction in the percentage of vehicles entering during the red-signal indication. These results should be viewed cautiously, however, since the treatment sites only included two intersections and since there was no indication of comparison or control sites.
R. A. van der Horst (50) found that increases in the yellow interval decreased the amount of red-signal violations. He conducted a behavioral before-and-after study at 23 urban and rural intersections in the Netherlands. One year after the yellow intervals were lengthened by 1 sec., the number of red-signal violations at the intersections lowered by one half. Bonneson's research indicates (39) that yellow increases in the range of 0.5 to 1.5 sec., that do not yield durations above 5.5 sec. can potentially reduce red-light running by about 50 percent.
Although lengthening the yellow interval may reduce signal violations, an interval that is too long could decrease the capacity of the intersection and increase the delay to motorists and pedestrians. Present thought is that longer intervals will cause drivers to enter the intersection later and it will breed disrespect for the traffic signal. The tendency for motorists to adjust to the longer interval and enter the intersection later is referred to as habituation.
The before-and-after study by Retting and Greene (47) evaluated the presence of habituation to the longer yellow interval by using a second after-period. The same after-period measurements (cycles with signal violation and late exits) were collected in a second after-period approximately six months after the first after-period. They were compared to the first afterperiod. The authors concluded that habituation to the longer yellow did exist although it may have been only largely present at one site for signal violations. No significant habituation was observed for late exits. In the before-and-after study at the two intersections in Arizona, Wortman et al. (49) compared plots of the time of entry of vehicles into the intersection. The researchers observed an increase in the number of drivers entering towards the end of the interval, possibly due to the lengthened yellow interval.
Additional research is needed to further understand the effect of lengthening the yellow interval on driver behavior.
The goal of traffic engineers has been to find an optimum interval for the yellow change and all-red (if used) while recognizing that there are traffic and roadway variables that must be considered. The Manual of Traffic Signal Design (45) cautions that change intervals greater than 6 sec. should be examined critically before being implemented. They cite loss in efficiency and capacity at the intersection and a tendency for local drivers to use more of the change interval when they know that it is longer than normal.
An all-red interval is that portion of a traffic signal cycle where all approaches have a red-signal display. If used, the all-red interval follows the yellow-change interval and precedes the next conflicting green interval. The purpose of the all-red interval is to allow time for vehicles that entered the intersection during the yellow-change interval to clear the intersection before the traffic-signal display for the conflicting approaches turns to green.
In many states, it is legal to enter the intersection during any portion of the yellow interval. Hence, if a vehicle enters the intersection at the end of the yellow interval and if an all-red interval is not provided, the vehicle will be in the intersection while a conflicting approach receives the green signal. Hence, there is a potential for a crash, even when no one entered the intersection illegally.
It should be pointed out that providing an all-red interval (or the length of the all-red interval) does NOT affect the decision or the act of the motorist in running the red-light. Because use of the red-clearance interval has been shown to increase the safety of an intersection, it is mentioned as a countermeasure in this toolbox because it can impact the safety of a signalized intersection.
As stipulated in the MUTCD, the all-red clearance interval is optional. The duration of the all-red interval shall be predetermined, which means the length of the interval should be calculated based on known intersection conditions and the length of time of the interval should not vary on a cycle-by-cycle basis. The MUTCD also stipulates that the duration of the all-red interval should not exceed 6 sec. There are no guidelines in the MUTCD for when the all-red interval should or may be used. For most agencies, the decision to use an all-red interval is tied to the determination of the yellow-change interval. In the latest version of ITE's Traffic Engineering Handbook (44) it is suggested that when the calculated change interval is greater than 5 sec., an all-red interval provides the additional time beyond 5 sec. Many agencies allocate the third term of Equation , shown previously, as the all-red interval.
While the use of an all-red interval is optional, survey results support that most jurisdictions use it at a majority of their intersections. In response to a survey conducted by The Urban Transportation Monitor (51), of the 76 city traffic engineers that responded, approximately 80 percent indicated that they use all-red at all signals and 20 percent indicated that they use it at some signals. The survey results indicate that only 35 percent apply the same standard interval length for applications. Those standard intervals ranged from 0.5 to 2 sec.
Studies have shown that providing a red interval does have a positive effect on the safety of the intersection. Four studies, summarized in Chapter 5 of the Synthesis of Safety Research Related to Traffic Control and Roadway Elements-Volume 1 (52), investigated the effect of adding an all-red interval on intersection crashes. All studies were performed in the 1970s in various states and cities and all of the study results indicated more than a 40 percent reduction in rightangle accidents at the study locations. These results, however, should be viewed cautiously because the study summaries did not indicate that measures were taken to control for trends and regression to the mean bias.
A positive safety benefit was reported in a more recent study by Datta et al. (53). Several improvements were made to three intersections in Detroit including the use of an all-red interval (1.5 to 2.0 seconds). Based on the results of a statistical analysis of a multi-year beforeand- after study of crashes, they concluded that there was a significant reduction in right-angle crashes and injuries after implementation of all-red intervals. This reported reduction in crashes could not be attributed solely to the all-red interval since other improvements were made. However, it does support that reductions in crashes can be realized from a combination of improvements identified in this toolbox including the use of an all-red interval.
The drawback to using an all-red is that it takes away from the green available for other movements and hence reduces the capacity of the intersection. The amount of reduction is dependent upon the number of phases and cycle length. For example, for a simple twophase signal with a 120-sec. cycle length timing plan, the reduction in capacity is only 2.5 percent (compared to the same signal without an all-red) from the addition of a 1.5 sec. all-red after each phase. The reduction is small, however in congested corridors, its use would exacerbate delays.
The "dilemma zone" has been defined recently to be the area in which it may be difficult for a driver to decide whether to stop or proceed through an intersection at the onset of the yellow-signal indication (54). It is also referred to as the "option zone" or the "zone of indecision" (55). One potential countermeasure to reduce red-light running is to reduce the likelihood that a vehicle will be in the dilemma zone at the onset of the yellow interval. This can be accomplished by placing vehicle detectors at the dilemma zone. They detect if a car is at the dilemma zone immediately before the onset of the yellow interval. If a vehicle is there, the green interval can be extended so that the vehicle can travel through the dilemma zone and prevent the onset of the yellow while in the dilemma zone. When combined with a speed detector, the countermeasure is even more beneficial. This countermeasure is referred to as dilemma-zone protection or green extension systems.
Zegeer and Deen conducted a before-and-after evaluation of green extension systems at three intersections in Kentucky to determine their effect on crashes (56). The duration of the before-period was 8.5 years and the duration of the after-period was 3.7 years. There were 70 accidents in the before-period and 14 accidents in the after-period. The authors found a 54 percent reduction in accidents per year at the three sites combined. No comparison or control groups were used. McCoy and Pesti conducted an evaluation of dilemma zone protection in Nebraska (55) as part of an evaluation of active advance-warning signs (discussed in a previous section). Dilemma-zone protection using conventional detectors was compared to dilemma-zone protection with active advance-warning signs in a cross-sectional evaluation. Overall, the two methods performed similarly when red-signal violations were the measure of effectiveness.
The final group of solutions to red-light running as described in the introduction of this chapter involves eliminating the need to stop. This can be done by removing the signal or redesigning the traditional intersection. An intersection should be designed following standards and guidelines found in AASHTO's A Policy on Geometric Design of Highways and Streets (46). Other design guidelines can be found in two ITE publications: The Traffic Safety Toolbox: A Primer on Traffic Safety (54) and Traffic Engineering Handbook (44).
Eliminate Need to Stop Through:
If there is a high incidence of red-light running violations, this may be because the traffic signal is perceived as not being necessary and does not command the respect of the motoring public. The decision to install a traffic signal is based on the traffic volume of the intersecting streets, pedestrian traffic and the flow of traffic through a network. Warrants are provided in the MUTCD that define the minimum conditions at which installing a traffic signal may be justified. However, sometimes signals are installed for reasons that dissipate over time. For instance, traffic volume may decrease due to changing land-use patterns or the creation of alternative routes. There have been studies of the impact of removing traffic signals and converting the intersection to STOP sign control. Kay et al. (57) found that at 26 intersections converted to multi-way stop control, there was a decrease in the average annual accident frequency of more than one accident per year. Where signals were replaced by two-way stop control, they found that on average there was an increase in rightangle crashes, but it was offset in the number of collisions and injuries by a reduction in rear-end crashes.
One of the primary factors that caused an increase in overall crashes was the presence of inadequate cornersight distance. They also found that one-way intersections with low volumes experience an overall crash reduction. This was the same finding in a study of 199 low-volume Philadelphia intersections, where it was determined that traffic-signal removal resulted in a 24 percent crash reduction (58).
Kay et al. (57) concluded that replacing unjustified signals with two-way stop control has the following beneficial impacts:
The removal of a traffic signal should be based on an engineering study. Factors to be considered in such a study are enumerated in Figure 3-22 from ITE's Traffic Control Devices Handbook (59). Specific guidance on signal removal can be found in Kay et al.'s report (57). Once it is established that a signal can be removed, Section 4B.02 of the MUTCD suggests a five-step process for removal of the signal.
Figure 3-22. Factors to Consider in Signal Removal
A "modern" roundabout can be described as a circulatory intersection that features channelized approaches, yields control for vehicles entering the circle and geometric curvatures that promote lower speeds within the roundabout. Other features include a central island off-limits to pedestrians and raised "splitter" islands on approach legs that divide entering and exiting traffic, as well as provide a refuge for pedestrians. Figure 3-23 shows a roundabout in Colorado.
With respect to the topic of this report, the roundabout replaces the traffic signal and obviously eliminates the red-light running problem. Assuming the roundabout is operationally more efficient (which may not be the case for many intersection conditions), the issue is whether or not it is a safer intersection considering all crashes.
Currently there are no recommended criteria or guidelines for when a roundabout should be considered. Roundabouts are acknowledged in the latest edition of AASHTO's A Policy on Geometric Design of Highways and Streets (46), with sparse design criteria, and there is guidance for appropriate pavement marking and signage in the MUTCD (see Section 3B.24). However, the most comprehensive guidance from planning to design is found in a FHWA document called Roundabouts: An Informational Guide (60).
Although use of roundabouts is limited in the United States, they are commonly used internationally. Many international studies have found that roundabouts greatly reduce the number of accidents and severity of accidents at converted intersections. Other benefits of roundabouts include: reduction in vehicle emissions, noise, fuel consumption and traffic delays, as well as eliminating the need for maintenance and electrical costs of operating a signalized intersection. The center island also provides a good location for landscaping and architectural treatments for improving the aesthetic quality of the intersection. Particular sites appropriate for roundabouts include locations with heavy delay on the minor road, an intersection with heavy left-turning traffic, an intersection with more than four legs or unusual geometry and intersections where U-turns are desirable (61).
There are certain drawbacks associated with roundabouts. The biggest drawback is to pedestrians, as they are limited to cross only on the approach legs and have no exclusive right of way (i.e. no pedestrian phase). Roundabouts are not appropriate everywhere as they do require a fair amount of right-of-way (outer diameter of approximately 100 ft.) and have a limited vehicle capacity. Public support of roundabouts in the United States is also a concern.
Conversion of a signalized intersection to a roundabout eliminates red-light running because the signal has been removed. The real test of safety effectiveness is in terms of total crashes. Aconversion from traffic-signal control to roundabouts reduces the total number of injury crashes by 30 to 40 percent (62). Another study states that left-turn accidents are eliminated and angle accidents are reduced by 80 percent (61). Such reductions can be attributed to the reduction in the number of conflict points and induced slower speeds through the intersection.
During periods of low traffic volumes no one should have to wait needlessly at a traffic signal. Today's modern traffic-signal control technologies are fully traffic actuated/adaptive systems that incorporate advanced loop or video detection methods. If working properly, even minor street traffic may not necessarily have to face a stopped condition. Yet, there are locations that are not instrumented to take advantage of these advanced technologies. Therefore, during low volume conditions at an intersection, it may be appropriate for signalized traffic-control devices to operate on flashing mode. The Traffic Control Devices Handbook (59) lists the following benefits associated with flashing mode operation:
By using the flashing mode, the need to stop (and/or wait) at an intersection is greatly reduced.
When in flashing mode, the MUTCD recommends a flashing yellow on the major street approaches and a flashing red on the minor street approaches. A less common arrangement (although common in California) is to use flashing red on all approaches to the intersection. The MUTCD also provides further requirements regarding the application and operation of traffic-control signals including the transition from steady to flashing mode and the need for flashing mode capability for emergency situations (see MUTCD Section 4D.11 and 4D.12).
The Traffic Control Devices Handbook cautions that the accident pattern at the intersection should be monitored to determine if the flashing mode has caused an increase in accidents. The indicators mentioned include the following three points:
These conditions were developed as a result of a FHWA study investigating different effects of flashing signal traffic control on intersection operation and safety concluded in 1980. After studying data from across the country, the study concludes that flashing yellow/red operation significantly increases the hazard of nighttime driving. The major exception is an intersection where the ratio of major street volume to minor street volume is greater than three, or where the major-street two-way volume is less than 200 vehicles per hour during flashing operation (63).
The potential hazardousness of using flashing operations was recently confirmed by Polanis (64) in Winston-Salem, NC. At 19 intersections where lowvolume flashing operations were removed (i.e., returned to normal signal control), right-angle crashes declined at every intersection (of which 16 had statistical significance), and for all the intersections combined there was a 78 percent reduction. Total accidents decreased by 33 percent, but for four locations an increase was observed. A follow-up after analysis showed that the right-angle crash reduction was sustained over a longer period. Polanis uses this finding to conclude that the use of flashing operation during low volume periods "...is a strategy to reduce delay that need not be abandoned, but its use requires careful application and additional monitoring."
There are a number of factors or reasons that cause drivers to run red-lights. There are also a number of countermeasures that can address these factors and discourage red-light running. The engineering countermeasures discussed in this chapter and summarized in Figure 3-24 address signal visibility/conspicuity, increasing the likelihood of stopping, removing the reasons for intentional violations and eliminating the need to stop. Most of these actions are low cost countermeasures. However, specific unit costs were not provided here since these costs can vary considerably among jurisdictions.
It is very difficult to prioritize countermeasures based on a relative estimate of cost effectiveness or crash reduction potential for a number of reasons.
Information provided from past studies that investigated the effectiveness of measures is limited. Additionally, the results of such studies are site specific. Moreover, the best countermeasure cannot be determined strictly from the effectiveness potential but must be appropriate for the specific intersection. For example, although modifying the yellow interval has been shown to reduce violations, the most appropriate countermeasure for a section with horizontal alignment problems is to provide warning of the upcoming signal.
Selecting the best countermeasure to use depends on individual site characteristics. The countermeasure most suited to the specific intersection can only be determined after conducting an engineering study that investigates the safety of the intersection as related to red-light running and also the occurrence of red-light violations. Additionally, an engineering study will investigate the existing design elements of the intersection. After such an investigation, the appropriate countermeasure for the specific site can be identified. Chapter 4 provides further information on conducting an engineering study.
Improve Signal Visibility
Increase Likelihood of Stopping
Eliminate Need to Stop
Improve Signal Conspicuity
Address Intentional Violations
|Figure 3-24. Summary of Engineering Countermeasures by Category.|
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