Handbook for Designing Roadways for the Aging Population

Chapter 2. Intersections

This section of the Handbook provides treatments for 16 different design elements in order to accommodate the needs and enhance the performance of road users with age-related diminished capabilities as they approach and negotiate intersections. Also, after the last element, eight additional promising practices are provided. Drawings are for illustrative purposes only; they are not to scale and should not be used for design purposes.

Proven Practices

1. Intersecting Angle (Skew)
2. Receiving Lane (Throat) Width
3. Channelization
4. Intersection Sight Distance
5. Offset Left-Turn Lanes
6. Delineation of Edge Lines and Curbs
7. Curb Radius
8. Left-Turn Traffic Control for Signalized Intersections
9. Right-Turn Traffic Control for Signalized Intersections
10. Street Name Signs
11. Stop and Yield Signs
12. Lane Assignment on Intersection Approach
13. Traffic Signals
14. Intersection Lighting
15. Pedestrian Crossings
16. Roundabouts

Promising Practices

17. Right Turn Channelization Design
18. Combination Lane-Use/Destination Overhead Guide Signs
19. Signal Head Visibility
20. High Visibility Crosswalks
21. Supplemental Pavement Markings for Stop and Yield Signs
22. Reduced Left-Turn-Conflict Intersections
23. Accessible Pedestrian Signal (APS) Treatments
24. Flashing Yellow Arrow

The single greatest concern in accommodating aging road users, both drivers and pedestrians, is the ability of these persons to negotiate intersections safely. For at least two decades, safety experts have keyed on relationships of age and road user type (driver or pedestrian) to understand injury and fatal crash experience at intersections in the United States. The findings of one widely cited analysis of nationwide crash data (Hauer, 1988) are shown in Figure 2.

The figure illustrates that aging drivers and aging pedestrians are involved in a higher percentage of injury and fatal incidents at intersections.

These same relationships endure today. A higher proportion of passenger vehicle drivers age 70 and older are involved in fatal multiple-vehicle crashes at intersections (40%) than drivers younger than age 70 (22%) (IIHS, 2007). While the number of fatalities, both overall and among aging road users, has declined in recent years, people age 65 and older are still involved in a disproportionate share of fatalities compared to the population as a whole (see Table 5). The number of aging drivers has increased substantially, making up a larger share of the driving population. Therefore, while people age 65 and older make up about 13 percent of the nation's population, they represent about 16 percent of drivers, 16 percent of driver fatalities, and 19 percent of pedestrian fatalities (FARS, 2013). These findings reinforce a long-standing recognition that driving situations involving complex speed-distance judgments under time constraints—the typical scenario for intersection operations—are more problematic for aging road users than for their younger counterparts. Other studies within the large body of evidence showing dramatic increases in intersection crash involvements as driver age increases have associated specific crash types and vehicle movements with particular age groups, linked in some cases to the driving task demands for a given maneuver (Campbell, 1993; Council and Zegeer, 1992; Staplin and Lyles, 1991).

Table 5. Involvement of the Older Population in Traffic Fatalities by Gender, 2002 and 2011 (FARS, 2013)

	2002			2011			Percentage Change, 2002–2011
	Total	Age 65+	Percentage of Total	Total	Age 65+	Percentage of Total	Total	Age 65+
Population (Thousands)
Total	287,625	35,522	12.4	311,592	41,394	13.3	8	17
Male	141,231	14,764	10.5	153,291	17,943	11.7	9	22
Female	146,395	20,758	14.2	158,301	23,451	14.8	8	13
Drivers Involved in Fatal Crashes
Total*	58,113	6,323	10.9	43,668	5,469	12.5	-25	-14
Male	42,377	4,340	10.2	31,806	3,769	11.8	-25	-13
Female	14,999	1,982	13.2	11,209	1,700	15.2	-25	-14
Driver Fatalities
Total*	26,659	3,984	14.9	20,753	3,402	16.4	-22	-15
Male	19,859	2,704	13.6	15,868	2,316	14.6	-20	-14
Female	6,799	1,279	18.8	4,881	1,086	22.2	-28	-15
Total Traffic Fatalities
Total*	43,005	6,680	15.5	32,367	5,401	16.7	-25	-19
Male	29,466	3,840	13.0	22,860	3,261	14.3	-22	-15
Female	13,529	2,838	21.0	9,499	2,140	22.5	-30	-25
Occupant Fatalities
Total*	37,375	5,541	14.8	27,060	4,417	16.3	-28	-20
Male	25,491	3,154	12.4	19,053	2,617	13.7	-25	-17
Female	11,875	2,386	20.1	8,000	1,800	22.5	-33	-25
Pedestrian Fatalities
Total*	4,851	1,064	21.9	4,432	845	19.1	-9	-21
Male	3,298	619	18.8	3,086	522	16.9	-6	-16
Female	1,552	444	28.6	1,345	323	24.0	-13	-27
Passenger Vehicle Occupant Fatalities
Total*	32,843	5,314	16.2	21,253	3,970	18.7	-35	-25
Male	21,431	2,959	13.8	13,811	2,208	16.0	-36	-25
Female	11,403	2,354	20.6	7,435	1,762	23.7	-35	-25
Motorcyclist Fatalities
Total	3,270	96	2.9	4,612	301	6.5	41	214
Male	2,961	89	3.0	4,181	285	6.8	41	220
Female	309	7	2.3	431	16	3.7	39	129
Pedalcyclist Fatalities
Total	665	56	8.4	677	90	13.3	2	61
Male	592	53	9.0	578	82	14.2	-2	55
Female	73	3	4.1	99	8	8.1	36	167

* Total includes unknown gender.

Complementing crash analyses and observational studies with subjective reports of intersection driving difficulties, a statewide survey of 664 aging drivers by Benekohal, et al. (1992) found that the following activities become more difficult for drivers as they grow older (with proportion of drivers responding in parentheses):

• Reading street signs in town (27 percent).
• Driving across an intersection (21 percent).
• Finding the beginning of a left-turn lane at an intersection (20 percent).
• Making a left turn at an intersection (19 percent).
• Following pavement markings (17 percent).
• Responding to traffic signals (12 percent).

Benekohal et al. (1992) also found that the following highway features become more important to drivers as they age (with proportion of drivers responding in parentheses):

• Lighting at intersections (62 percent).
• Pavement markings at intersections (57 percent).
• Number of left-turn lanes at an intersection (55 percent).
• Width of travel lanes (51 percent).
• Concrete lane guides (raised channelization) for turns at intersections (47 percent).
• Size of traffic signals at intersections (42 percent).

Comparisons of responses from drivers ages 66–68 versus those age 77 and older showed that the older group had more difficulty following pavement markings, finding the beginning of the left-turn lane, and driving across intersections. Similarly, the level of difficulty for reading street signs and making left turns at intersections increased with age. Turning left at intersections was perceived as a complex driving task. This was made more difficult when raised channelization providing visual cues was absent, and only pavement markings designated which were through lanes versus turning lanes ahead. For the oldest age group, pavement markings at intersections were the most important item, followed by the number of left-turn lanes, concrete guides, and intersection lighting. A study of aging road users completed in 1996 provides evidence that the single most challenging aspect of intersection negotiation for this group is performing left turns during the permissive signal phase (Staplin, et al., 1997).

During focus group discussions (Benekohal et al., 1992), aging drivers reported that intersections with too many islands are confusing; raised curbs that are unpainted (unmarked) are difficult to see; and textured pavements (rumble strips) are of value as a warning of upcoming raised medians, approaches to (hidden or flashing red) signals, and the roadway edge/shoulder lane boundary. Study subjects indicated a clear preference for turning left on a protected arrow phase, rather than making permissive-phase turns. When turning during a permissive phase of signal operation, they reported waiting for a large gap before making a turn, which frustrates drivers behind them. A key finding was the need for more time to react.

Additional insight into the problems aging drivers experience at intersections was provided by focus group responses from 81 aging drivers (Staplin et al., 1997). The most commonly reported problems are listed below:

• Difficulty in turning their heads at skewed (non-90-degree) angles to view intersecting traffic.
• Difficulty in smoothly performing turning movements at tight corners.
• Hitting raised concrete barriers such as channelizing islands in the rain and at night.
• Finding oneself positioned in the wrong lane—especially a "turn-only" lane—during an intersection approach, due to poor visibility (maintenance) of pavement markings or the obstruction of roadside signs designed to inform drivers of intersection traffic patterns.
• Difficulty at the end of an auxiliary (right) turn lane in seeing potential conflicts well and quickly enough to smoothly merge with adjacent-lane traffic.
• Merging with adjacent-lane traffic at a pavement width reduction, when the lane drop occurs near (i.e., within 500 ft of) an intersection.

For aging pedestrians, age-related diminished capabilities may make it more difficult to negotiate intersections. Aging pedestrians face a variety of concerns, including decreased visual acuity, increased risk of falls, slowed walking and crossing speeds, and decreased ability to judge safe gaps and avoid turning vehicles.

Lighting and visibility at intersections are increasingly important to pedestrians as they age. In a survey of aging pedestrians (average age of 75) involved in crashes, 63 percent reported that they failed to see the vehicle that hit them, or to see it in time to take evasive action (Sheppard and Pattinson, 1986). Knoblauch, et al. (1995) noted that difficulty seeing a vehicle against a (complex) street background may occur with vehicles of certain colors, causing them to blend in with their background. Reductions in visual acuity make it more difficult for aging pedestrians to read the crossing signal as well (Bailey, et al., 1992).

With increasing age, there is a concurrent loss of physical strength, joint flexibility, agility, balance, coordination and motor skills, and stamina. These losses contribute to difficulty negotiating curbs and an increased risk of falling as a result of undetected surface irregularities in the pavement and inaccurate estimation of curb heights (Clark, Lord, and Webster, 1993).

The physical limitations of aging pedestrians result in a greater likelihood to delay before crossing; to wait for longer gaps between vehicles before attempting to cross the road (Tobey, Shungman, and Knoblauch, 1983); to spend more time at the curb; to take longer to cross the road (Hoxie and Rubenstein, 1994; Knoblauch, Nitzburg, Dewar, et al., 1995); and to make more head movements before and during crossing (Wilson and Grayson, 1980). Parsonson (1992) reported that the State of Delaware has found that pedestrians do not react well to the short WALK and long flashing DONT WALK timing pattern.

Turning vehicles are also a concern for aging pedestrians. The loss of peripheral vision and "useful field of view" increases an aging pedestrian's chances of not detecting approaching and turning vehicles from the side. The analysis by Council and Zegeer (1992) included an examination of vehicle-pedestrian crashes and the collision types in which aging pedestrians were over-involved. The results showed aging pedestrians to be overrepresented in both right- and left-turn crashes. The young-elderly (ages 65-74) were most likely to be struck by a vehicle turning right, whereas the old-elderly (age 75 and older) were more likely to be struck by a left-turning vehicle.

Together, these findings from research on aging road users reinforce the overriding design principles to "clarify" and 'simplify" traffic operations at intersections. By providing appropriate advance information about route choices and destinations, clearly identifying lane assignments for allowed maneuvers, and implementing conspicuous and easily comprehensible sign and signal displays for traffic control, engineers can manage workload during intersection approach and negotiation in a manner that benefits road users of all ages. Likewise, the need for intersection geometrics to unambiguously convey path, direction, and speed is universal, and such "positive guidance" is an explicit goal of the treatments presented in this chapter.

Proven Practices
1. Intersecting Angle (Skew)

The HSM (page 10-32) defines skew angle as: intersection skew angle (in degrees); the absolute value of the difference between 90 degrees and the actual intersection angle.

A. Unrestricted Right-of-Way

In the design of new facilities or redesign of existing facilities where right-of-way is not restricted, all intersecting roadways should meet at a 90-degree angle (as indicated in Figure 3).

References *: Green Book:1, NCHRP 279:1, TEH:1, NCHRP 500-9:1

B. Restricted Right-of-Way

In the design of new facilities or redesign of existing facilities where right-of-way is restricted, intersecting roadways should meet at an angle of not less than 75 degrees (as indicated in Figure 4).

References *: TEH:2

C. Skewed Signalized Intersections

At skewed signalized intersections where the approach leg to the left intersects the driver's approach leg at an angle of less than 75 degrees, prohibit right turn on red (RTOR) (see Figure 5).

References *: TEH:4, MUTCD:2

The rationale and supporting evidence for these treatments begins on page 96 of the Handbook.

2. Receiving Lane (Throat) Width

A. Minimum Width

Wherever practical, a minimum receiving throat width of 16 ft is recommended. The total width may include a travel lane of 11 to 12 ft and a paved shoulder or bicycle lane of 4 to 5 ft as shown in Figure 6.

References *: NCHRP 279:2, TEH:2

The rationale and supporting evidence for this treatment begins on page 99 of the Handbook.

3. Channelization

A. Left- and Right-Turn Lanes

Raised channelization with sloping curbs (see Figure 7) is recommended over channelization accomplished through the use of pavement markings alone (flush) for left- and right-turn lane treatments at intersections on all roadways with operating speeds of less than 45 mph.

References *: Green Book:4, NCHRP 279:4, TEH:4, MUTCD:4, NCHRP 500-9:4

B. Retroreflective Markings

Where raised channelization is implemented at intersections (see Figure 7) the median and island curb sides and curb horizontal surfaces should be treated with retroreflectorized markings, such as edge lines, painted curbs, or raised pavement markers, and be maintained at a minimum luminance contrast level* as follows:

• B-1. With overhead lighting, a contrast of at least 2.0.
• B-2. Without overhead lighting, a contrast of at least 3.0.

Contrast should be calculated according to this formula:

* Luminance is the amount of light reflected from an object. This is different from retroreflectivity, which is a property of a material. While increasing retroreflectivity generally results in higher luminance, (often described as brightness)—especially at night—this may vary greatly for the same object or marking depending upon such factors as the location and intensity of the source of illumination, and the angle at which a driver views it.

References *: MUTCD:4, NCHRP 500-9:4

C. Acceleration Lane

If right-turn channelization is present at an intersection, an acceleration lane providing for the acceleration characteristics of passenger cars as delineated in AASHTO (2011) specifications is recommended for operating speeds of 45 mph or greater.

References *: Green Book:4

D. Sloping vs. Vertical Curbs

The use of sloping curbs rather than vertical curbs (see Figure 7) for channelization is recommended, except where the curbs surround a pedestrian refuge area or are being used for access control. Vertical curbs should also not be used for channelization on high-speed (i.e., 45 mph or greater) roadways.

References *: Green Book:4, NCHRP 279:4

E. Pedestrian Refuge Island

If right-turn channelization is present and pedestrian traffic may be expected based on surrounding land use, it is recommended that an adjacent pedestrian refuge island, conforming to MUTCD (2009) and AASHTO (2011) specifications, be provided.

References *: Green Book:1, NCHRP 279:3, MUTCD:1

F. Median Channelization

To reduce unexpected midblock conflicts with opposing vehicles, the use of channelized left-turn lanes in combination with continuous raisedcurb medians is recommended instead of center, two-way, left-turn lanes (TWLTL) for new construction or reconstruction where average daily traffic volumes exceed 20,000 vehicles per day, or for remediation where there is a demonstrated crash problem, or wherever a need is demonstrated through engineering study.

References *: Green Book:4, NCHRP 279:4

The rationale and supporting evidence for these treatments begins on page 102 of the Handbook.

4. Intersection Sight Distance

A. Gap Value

It is recommended that a gap of no less than 8.0 s, plus 0.5 s for each additional lane crossed, be used in intersection sight distance (ISD) calculations to accommodate the slower decision-making and maneuver times of aging drivers for the following cases:

• Cases B1, B2, and B3 – stop-control on the minor road,
• Cases C1 and C2 – yield control on the minor road,
• Case D – signalized with permissive left-turn phases and/or where RTOR is permitted and/or which are placed on flashing operations at night, and
• Case F – left turns from a major roadway.

Note: Cases refer to the intersection control cases defined in the AASHTO Green Book (2011).

References *: Green Book:1

The rationale and supporting evidence for these treatments begins on page 107 of the Handbook.

5. Offset Left-Turn Lanes

A. Full Offset – Opposing Cars

Left-turn lanes should be positively offset (as shown in Figure 8) at least 4 ft to the left of the opposing left-turn lane to achieve the desired sight distance for the left-turning driver. This will provide a margin of safety for aging drivers who, as a group, do not position themselves to the far left within the lane and within the intersection before initiating a left turn.

References *: Green Book:1, NCHRP 279:4, TEH:4, NCHRP 500-9:4

B. Full Offset – Opposing Heavy Trucks

At intersections where engineering judgment indicates a high probability of heavy trucks as the opposing left-turning vehicles, the positive offset is recommended to be 5.5 ft to achieve the desired sight distance.

References *: Green Book:4, NCHRP 279:4, TEH:4, NCHRP 500-9:4

C. Minimum Offset

At locations where the full offset distances cannot be obtained, it is recommended that the minimum offset distances shown in Table 6 be provided to achieve minimum required sight distances according to design speed. It is recommended that the "Opposing Truck" values be used where the opposing left-turn traffic includes a moderate to heavy volume of large trucks.

References *: Green Book:4, NCHRP 279:4, TEH:4, NCHRP 500-9:4

Table 6. Minimum offset distances for left-turn lanes.

Design Speed (mph)	Minimum Offset (ft)
	Opposing Car	Opposing Truck
≤30	0.8	3.0
35	1.4	3.5
40	1.8	3.8
45	2.1	4.1
50	2.4	4.2
55	2.6	4.4
60	2.7	4.5
65	2.8	4.6
70	2.9	4.7

1 ft = 0.305 m

D. Signs and Markings

At intersections where the left-turn lane treatment results in channelized offset left-turn lanes (e.g., a parallel or tapered left-turn lane between two medians), the following countermeasures (see Figure 9) are recommended to reduce the potential for wrong-way maneuvers by drivers turning left from a stop-controlled intersecting minor roadway:

• D-1. Largest practical sign sizes as specified in the MUTCD (2009) for DIVIDED HIGHWAY CROSSING, WRONG WAY, DO NOT ENTER, KEEP RIGHT, and ONE WAY signs.

  References *: TEH:4, MUTCD:3, NCHRP 500-9:4

• D-2. For the signs listed above, use prismatic retroreflective sheeting, to provide increased sign conspicuity and legibility for older drivers. Ensure these signs are replaced before they meet the minimum sign retroreflectivity levels, which includes white on red signs having a contrast ratio of at least 3:1.

  References *: MUTCD:1, NCHRP:4

• D-3. Retroreflective lane-use arrows.

  References *: MUTCD:1, NCHRP 500-9:4

• D-4. Retroreflective pavement marking extensions of the center line that scribe a path through the turn, except where extensions for opposing movements cross.

  References *: MUTCD:3, NCHRP 500-9:4

• D-5. Placement of 23.5-ft- long retroreflective wrong-way arrows in the through lanes at locations determined to have a special need, as specified in the MUTCD (2009), Sections 3B.19 and 2E-50.

  References *: MUTCD:3, NCHRP 500-9:4

• D-6. Delineation of median noses using retroreflective treatments to increase their visibility and improve driver understanding of the intersection design and function.

  References *: Green Book:1, MUTCD:2, NCHRP 500-9:4

E. Pedestrian Accommodations

At intersections where there are high pedestrian volumes, and the offset left-turn treatment results in a crossing width that would require a pedestrian walking at 3.0 ft/s to cross in two stages, the following is recommended to create a pedestrian crossing island (or refuge area), as shown in Figure 10:

• E-1. Flush (painted) channelization is used to separate the left-turn lane and adjacent through lanes.
• E-2. Raised channelization with a vertical curb and a minimum width of 6 ft is used to separate the left-turn lane from opposing travel lanes. While a 6-ft width is the minimum, there are advantages to providing wider islands where possible. Sloped curbs should be used instead of vertical curbs for channelization on high-speed (i.e., 45 mph or greater) roadways.

References *: Green Book:1, MUTCD:2

The rationale and supporting evidence for these treatments begins on page 122 of the Handbook.

6. Delineation of Edge Lines and Curbs

A. Visibility

A minimum in-service luminance contrast level between the marked edge of the roadway and the road surface should be maintained as follows:

• A-1. At intersections with overhead lighting, a contrast of 2.0 or higher.
• A-2. At intersections without overhead lighting, a contrast of 3.0 or higher.

Contrast should be calculated according to this formula:

* Luminance is the amount of light reflected from an object. This is different from retroreflectivity, which is a property of a material. While increasing retroreflectivity generally results in higher luminance, (often described as brightness)—especially at night—this may vary greatly for the same object or marking depending upon such factors as the location and intensity of the source of illumination, and the angle at which a driver views it.

References *: MUTCD:4, RLH:4

B. Intersection Curbs

Curbs at intersections (including median islands and other raised channelization) should be delineated on their vertical face and at least a portion of the top surface, in addition to the provision of a marked edge line on the road surface (see Figure 11).

The use of a Keep Right (R4-7 Series) or a Double Arrow (W12-1) sign with the addition of a low-mounted Type 1 Object Marker (OM1-1) near the median and channelizing island noses, respectively, could also be helpful to aging road users. These signs are optional and can be used if they are warranted. Since markings primarily supplement signing, this treatment should be placed in addition to the signing available for these locations.

References *: Green Book:1, MUTCD:1

The rationale and supporting evidence for these treatments begins on page 129 of the Handbook.

7. Curb Radius

A. Simple Radius

Where roadways intersect at 90 degrees and are joined with a simple radius curve, a corner curb radius in the range of 25 ft to 30 ft is recommended to: (a) facilitate vehicle turning movements, (b) moderate the speed of turning vehicles, and (c) avoid unnecessary lengthening of pedestrian crossing distances (see Simple Curve in Figure 12).

References *: AASHTO:1, NCHRP 279:1

B. Accommodation of Heavy Vehicles

When it is necessary to accommodate turning movements by large trucks, the use of offsets, tapers, and compound curves is recommended in place of larger simple radii (e.g., 75 ft or more) to minimize pedestrian crossing distances (see Figure 12).

References *: AASHTO:1, NCHRP 279:4, TEH:4

The rationale and supporting evidence for these treatments begins on page 132 of the Handbook.

8. Left-Turn Traffic Control for Signalized Intersections

A. Protected-Only Left-Turn Phasing

The use of protected-only left-turn operations is recommended for all left-turning movements, whenever appropriate. In particular, protected-only left-turn phasing should be considered where minimum intersection sight distance requirements are not achieved through the use of offset left-turn lanes (see Design Element 5) or other geometric design features, or where a pattern of permissive left-turn crashes occurs.

References *: Green Book:4, TEH:1, MUTCD:1

B. Permissive Left-Turn Signing

If circular green is used as the permissive indication of a protected/permissive left-turn, consistent use of the MUTCD R10-12 sign, (LEFT TURN YIELD ON GREEN ) is recommended, with overhead placement preferred at the intersection adjacent to the left-turn signal face (see Figure 13).

References *: TEH:4, MUTCD:1

C. Advance Signing

Where practical, an additional R10-12 sign (i.e., in addition to the R10-12 sign adjacent to the signal face) should be placed in advance of the intersection to advise left-turning drivers of permissive signal operation. The sign should be displayed at a 3-s preview distance before the intersection, or at the beginning of the left-turn lane, as per engineering judgment, accompanied by an AT SIGNAL (R10-31P) supplemental plaque as shown in Figure 14. [See time-speed-distance table on page 5.]

References *: AASHTO:3, MUTCD:1, NCHRP 500-9:4

D. Lead versus Lag Phasing

A leading protected left-turn phase is recommended wherever protected left-turn signal operation is implemented (as opposed to a lagging protected left-turn phase).

References *: TEH:2, MUTCD:2

The rationale and supporting evidence for these treatments begins on page 135 of the Handbook.

9. Right-Turn Traffic Control for Signalized Intersections

A. Turn Prohibition

At signalized intersections where a right turn on red is prohibited, a supplemental NO TURN ON RED sign, using the MUTCD R10-11 design as shown in Figure 15, should be placed at a location on either the near or opposite side of the intersection where, per engineering judgment, it will be most conspicuous. This supplemental NO TURN ON RED sign is in addition to the MUTCD recommended practice of installing an R10-11 series sign near the appropriate signal head.

References *: TEH:4, MUTCD: 1

B. Skewed Signalized Intersections

At skewed signalized intersections where the approach leg to the left intersects the driver's approach leg at an angle of less than 75 degrees (as illustrated in Figure 16), prohibit right turn on red (RTOR).

References *: TEH:4, MUTCD:1

C. Pedestrian Protection

The posting of MUTCD standard R10-15 signs, Turning Vehicles Yield to Pedestrians (shown in Figure 17) is recommended wherever engineering judgment indicates a clear potential for right-turning vehicles to come into conflict with crossing pedestrians. (Note that a yellow background color may be used instead of fluorescent yellow-green for this sign.)

References *: MUTCD:1

The rationale and supporting evidence for these treatments begins on page 148 of the Handbook.

10. Street Name Signs

A. Letter Heights and Sign Border

To accommodate the reduction in visual acuity associated with increasing age, minimum letter heights of 6 in for uppercase letters and 4.5 in for lowercase letters are recommended for use on ground-mounted street-name signs (MUTCD D3-1, as shown in Figure 18) on all roads where the posted speed limit is at or below 25 mph. On all roads where the posted speed limit is greater than 25 mph, letter heights of 8 in for uppercase letters and 6 in for lowercase letters should be used.

The use of overhead-mounted street-name signs is recommended at major intersections as a supplement to ground-mounted street-name signs. Minimum letter heights of 12 in for uppercase letters and 9 in for lowercase letters are recommended by the MUTCD. In the design of street-name signs, the use of larger letter heights may require a larger sign panel. The border may be eliminated on street-name signs if necessary to minimize sign panel size while accommodating the larger letter size.

References *: MUTCD:1, NCHRP 500-9:4

B. Advance Street-Name Plaque

Wherever an advance intersection warning sign is installed (MUTCD W2 series) it should be accompanied by an advance street name plaque (W16-8P or W16-8aP) using minimum letter heights of 6 in for uppercase letters and 4.5 in for lowercase letters (see Figure 19). Where an advance traffic control sign (MUTCD W3 series) is installed on a multi-lane approach, an advance street name plaque (W16-8P or W16-8aP), using the same minimum letter heights described above, should be considered.

References *: MUTCD:2, NCHRP 500-9:4

C. Advance Street-Name Sign

In the absence of an advance intersection warning sign or advance traffic control sign (with accompanying advance street-name plaque), the use of advance street-name signs (MUTCD D3-2) for major intersections is recommended, with turn bays to provide adequate preparation time for any lane change and/or turning maneuvers (see Figure 20).

References *: MUTCD:1, NCHRP 500-9:1

D. Directional Street-Name Sign

When different street names are used for different directions of travel on a crossroad, the names should be separated and accompanied by directional arrows on both advance and intersection street-name signs, as shown in Figure 20.

References *: MUTCD:1

E. Retroreflectivity

For ground-mounted street-name signs installed at intersections in areas of intensive land use, complex design features, and heavy traffic, prismatic retroreflective sheeting that provides for high retroreflectance should be used to provide increased sign conspicuity and legibility for aging drivers. The sheeting should be replaced well before it reaches the minimum levels designated in the current MUTCD (Section 2A.08 in the 2009 MUTCD).

References *: MUTCD:1

The rationale and supporting evidence for these treatments begins on page 152 of the Handbook.

11. Stop and Yield Signs

Treatments to improve the safe use of intersections by aging drivers, where the need for stop control or yield control has already been determined, include the following:

A. Sign Size

The use of standard size (30-in for single lane applications, 36-in for multi-lane applications) STOP (R1-1) and standard size (36-in for single lane applications, 48-in for multi-lane applications, 60-in for freeway applications) YIELD (R1-2) signs, as a minimum, is required by the 2009 MUTCD wherever these devices are implemented, with the option of using larger R1-1 (36-in for single lane applications, or 48-in in any location) signs where engineering judgment indicates that greater emphasis or visibility is required.

References *: MUTCD:1

B. Retroreflectivity

A minimum sign background (red area) retroreflectivity level (i.e., coefficient of retroreflection [RA]) for STOP (R1-1) and YIELD (R1-2) signs is as follows:

• B-1. 12 cd/lux/m² for roads with operating speeds lower than 40 mph.
• B-2. 24 cd/lux/m² for roads with operating speeds of 40 mph or higher.

Signs with an RA below these levels should be replaced.

References *: TEH:4, MUTCD:1

C. Supplemental Warning Sign

The use of a 30-in x 18-in supplemental warning sign panel (MUTCD W4-4P) as illustrated in Figure 21, mounted below the STOP (R1-1) sign, is recommended for two-way stop-controlled intersection sites selected on the basis of crash experience, where the sight triangle is restricted, and wherever a conversion from four-way stop to two-way stop operations is implemented.

References *: MUTCD:1

D. Location of Stop Ahead Sign

A STOP AHEAD sign (MUTCD W3-1, as shown in Figure 22) should be used where the distance at which the STOP sign is visible is less than the AASHTO stopping sight distance (SSD) at the operating speed, plus an added preview distance of at least 2.5 s. [See time-speed-distance table on page 5.]

References *: Green Book:4, TEH:4, MUTCD:1, NCHRP 500-9:4

E. Transverse Treatments and Supplemental Pavement Markings

Utilize supplemental pavement markings on approaches to stop-controlled or yield-controlled intersections where engineering judgment indicates a special need due to sight restrictions, high approach speeds, or a history of ran-stop-sign crashes. "STOP AHEAD" pavement markings to supplement STOP AHEAD signs and triangular pavement markings to supplement YIELD AHEAD signs are described in MUTCD Section 3B.20 and Figure 32. Transverse pavement striping or rumble strips may also be considered where high approach speeds are a concern.

References *: TEH:4, NCHRP 500-9:3

The rationale and supporting evidence for these treatments begins on page 160 of the Handbook.

12. Lane Assignment on Intersection Approach

A. Lane-Use Control Signs

The consistent overhead placement of lane-use control signs (MUTCD R3-5 and R3-6 series) at intersections on a signal mast arm or span wire is recommended, as illustrated in Figure 23.

References *: MUTCD:1

B. Advance Signs and Markings

The consistent posting of lane-use control signs (MUTCD R3 series) plus application of lane-use arrow pavement markings at a preview distance of at least 5 s (at operating speed) in advance of a signalized intersection is recommended, regardless of the specific lighting, channelization, or delineation treatments implemented at the intersection. [See time-speed-distance table on page 5.] R3-5 and R3-6 series signs should be mounted overhead wherever practical.

References *: MUTCD:4

The rationale and supporting evidence for these treatments begins on page 170 of the Handbook.

13. Traffic Signals

A. Visibility

To ensure visibility and conspicuity of the traffic signal, the following is recommended:

• A-1. A maintained performance level of 200 cd for peak intensity for both 8-in and 12-in signals.
• A-2. Use of 12-in signals in all cases except the few limited situations in which the MUTCD allows the use of 8-in signals.

References *: MUTCD:1

B. All-Red Clearance Interval

To accommodate age differences in perception-reaction time, an all-red clearance interval should be consistently implemented, with length determined according to the Institute of Transportation Engineers (2013) expressions given below:

• B-1. Where pedestrian traffic is prohibited, or no pedestrian crossing facilities are provided, use:
  

  B-2. Where pedestrian crossing facilities are provided, use:
  

  where:

  • r = length of red clearance interval, to the nearest 0.1 s.
  • W = width of intersection (ft), measured from the near-side stop line to the far edge of the conflicting traffic lane along the actual vehicle path.
  • P = width of intersection (ft), measured from the near-side stop line to the far side of the farthest conflicting pedestrian crosswalk along the actual vehicle path.
  • L = length of vehicle (recommended as 20 ft)
  • V = approach speed of the vehicle (mph)

References *: MUTCD:2

C. Backplates

Backplates with retroreflective borders should be considered as part of efforts to systemically improve safety performance at signalized intersections. Use backplates with traffic signals on all roads with operating speeds of 40 mph or higher. The use of backplates with signals is also recommended on roads with operating speeds lower than 40 mph where engineering judgment indicates a need due to the potential for sun glare problems, site history, or other variables. Yellow retroreflective borders, shown in Figure 24, may be used as an option to improve visibility of the illuminated face of the signal. The yellow retroreflective strip should have a minimum width of 1 inch and a maximum width of 3 inches and be placed along the perimeter of the face of a signal backplate to project a rectangular appearance at night. The yellow retroreflective strip should have a minimum width of 1 inch and a maximum width of 3 inches and be placed along the perimeter of the face of a signal backplate to project a rectangular appearance at night.

References *: MUTCD:4

The rationale and supporting evidence for these treatments begins on page 173 of the Handbook.

14. Intersection Lighting

A. Fixed Installations

Wherever feasible, fixed lighting installations are recommended as follows:

• A-1. Where the potential for wrong-way movements is indicated through crash experience or engineering judgment.
• A-2. Where twilight or nighttime pedestrian volumes are high.
• A-3. Where shifting lane alignment, turn-only lane assignment, or a pavement-width transition forces a path-following adjustment at or near the intersection.

References *: Green Book:4, RLH:4

B. Maintenance

Regular cleaning of lamp lenses, and lamp replacement when output has degraded by 20 percent or more of peak performance (based on hours of service and manufacturer's specifications), are recommended for all fixed lighting installations at intersections.

References *: RLH:4

The rationale and supporting evidence for these treatments begins on page 181 of the Handbook.

15. Pedestrian Crossings

A. Walking Speed

To accommodate the aging pedestrian who typically has a shorter stride, slower gait, and delayed 'start-up" time before leaving from a position further back from the curb at signalized crossings, the joint application of the following practices is recommended:

• A-1. Use a walking speed of 3.0 ft/s to calculate total crossing time (WALK interval plus pedestrian clearance interval).
• A-2. Measure crossing distance from a location 6 ft back from the curb or travel lane edge to the far side of the travel way being crossed.

References *: Green Book:2, NCHRP 279:2, MUTCD:1

B. Channelized Right-Turn Lane

For pedestrian crossings where the right-turn lane is channelized, it is recommended that:

• B-1. An adjacent pedestrian refuge island conforming to MUTCD (2009) and AASHTO (2011) specifications be provided.
• B-2. If a crosswalk is within the channelized area, it should be located approximately one car length from the yield line for the intersection (see Figure 25), which will allow drivers on the approach leg to look for and yield to pedestrians before reaching the intersecting roadway and scanning for gaps in traffic.

References *: Green Book:4, NCHRP 279:4, TEH:4, MUTCD:2

C. Educational Signs

Where engineering judgment deems there to be a need to improve understanding of pedestrian signals, it is recommended that educational signs be posted near the crosswalk as follows:

• C-1. For single stage crossings, use MUTCD R10-3b, R10-3c, R10-3e, R10-3f, R10-3g, or R10-3i as shown in Figure 26.
• C-2. For two-stage crossings using a median refuge island, use MUTCD R10-3d or R10-3h (see Figure 26) on the corners of the intersection and the placards defined above on the median refuge island.

References *: MUTCD: 2

D. Turning Vehicles Yield to Pedestrians Sign

The posting of the MUTCD R10-15 sign (see Figure 27) is recommended wherever engineering judgment indicates a clear potential for right-turning vehicles to come into conflict with crossing pedestrians.

References *: MUTCD:1

E. Leading Pedestrian Interval

At intersections with high turning-vehicle volumes and no turn on red (NTOR) control for traffic moving parallel to a marked crosswalk, a leading pedestrian interval (LPI), timed to allow slower walkers to cross at least one moving lane of traffic is recommended to reduce conflicts between pedestrians and turning vehicles. The length of the LPI, which should be at least 3 s, may be calculated using the formula:

where:

• LPI = seconds between onset of the WALK signal for pedestrians and the green indicator for vehicles.
• ML = width of moving lane in ft
• PL = width of parking lane (if any) in ft
• 6.0 = distance in ft back from the edge of the curb to the assumed starting location for pedestrians
• 3.0 = walking speed in ft/s

References *: MUTCD:1

F. Countdown Signal

Countdown pedestrian signals (see Figure 28) should be installed at all signalized intersections where pedestrian signals are warranted. The 2009 MUTCD requires the use of countdown pedestrian signals when the pedestrian change interval is greater than 7s.

References *: MUTCD: 1

The rationale and supporting evidence for these treatments begins on page 185 of the Handbook.

16. Roundabouts

There are features of roundabout intersections that can benefit aging drivers and be a beneficial treatment over a traditional stop- or signal-controlled intersection if properly designed to meet the needs of that location. When properly designed, roundabouts are low-speed intersections, which provide benefits to aging drivers and pedestrians alike. In addition, roundabouts effectively eliminate severe right-angle crashes. Any crashes that do occur are typically low-angle (i.e., sideswipe) crashes at reduced speeds. The low-speed features associated with roundabouts eliminate the problems associated with making unprotected left turns at intersections. It is therefore recommended that roundabouts be considered as part of the engineering study in the design of new intersections and the redesign of existing intersections.

Treatments for preferred practices when a State or local highway authority has determined through engineering study to install a modern roundabout during construction or reconstruction of an intersection include the following (see Figure 29):

A. Number of Lanes

Unless required by operational needs, it is recommended that roundabout installations be limited to one-lane entrances and exits and one lane of circulating traffic.

References *: NCHRP 672:1

B. Pedestrian Crossings

Pedestrian crossings at single-lane roundabouts should be set back a minimum of 25 ft from the yield lines and include a crossing island of at least 6 ft in width.

References *: NCHRP 672:1

C. Splitter Islands

To control for wrong-way movements, calm traffic, and provide a pedestrian refuge for all roundabout categories, raised splitter islands should be used, as opposed to pavement markings, to delineate the channelization. The pedestrian crosswalk area should be designed at street level (crosswalk cut through a splitter island).

References *: NCHRP 672:1

D. Conspicuity

To enhance the conspicuity of roundabouts in all categories, the sides and tops of curbs on the splitter islands and the central island should be treated with retroreflective markings, and be maintained at a minimum luminance contrast level* as follows:

• D-1. At roundabouts with overhead lighting, a contrast of 2.0 or higher.
• D-2. At roundabouts without overhead lighting, a contrast of 3.0 or higher.

Contrast should be calculated according to this formula:

* Luminance is the amount of light reflected from an object. This is different from retroreflectivity, which is a property of a material. While increasing retroreflectivity generally results in higher luminance, (often described as brightness)—especially at night—this may vary greatly for the same object or marking depending upon such factors as the location and intensity of the source of illumination, and the angle at which a driver views it.

References *: NCHRP 672:4

E. Advance Warning Sign

The use of an advance roundabout warning sign (W2-6), as shown in Figure 30, is recommended on all approaches to a roundabout.

References *: MUTCD:1

F. Directional Signs

The use of a Roundabout Directional Arrow sign (R6-4 series) is recommended to direct traffic counter-clockwise around the central island. This sign display should be placed on the central island in direct view of a driver's entry point, as shown in Figure 31, (if different than at the centerline of the approaching roadway).

References *: MUTCD:1

G. Roundabout Circulation Plaque

The Roundabout Circulation Plaque (R6-5P) should be placed immediately below the R1-2 Yield sign on both sides of the road at each entrance to a roundabout (see Figure 32).

References *: MUTCD:5

The rationale and supporting evidence for these treatments begins on page 197 of the Handbook.

Promising Practices

These are treatments being utilized by transportation agencies that should benefit aging road users as determined by a subjective assessment by staff participating on the development of this Handbook. Current trends indicate these practices have a positive impact on aging road user safety.

17. Right-Turn Channelization Design

Right-turn channelization with tighter turning radii to reduce turning speeds to approximately 17 to 18 mph, decrease pedestrian crossing distances, and optimize the right-turning motorists' line of sight should be considered during design, as shown in the Preferred example on the right of Figure 33. Designs such as those on the left of Figure 33 are potentially problematic as drivers have to turn their heads farther to see oncoming traffic. The short curve radius should be between 25 and 40 ft, and the long curve radius should be between 150 and 275 ft. Traffic control devices at the end of the channelization should be visible to vehicles entering the channelized lane.

References *: NCHRP 674:1

The rationale and supporting evidence for these treatments can be found beginning on page 215 of this Handbook.

18. Combination Lane-Use/Destination Overhead Guide Signs

At intersections where complex design features or heavy traffic is present, and specific guidance advising roadway users which lane is necessary for their intended destination, combination lane use/destination signs (D15-1) should be used. These signs are typically used as overhead combination lane use destination guide signs and are described in Section 2D.33 of the 2009 MUTCD (see Figure 34).

References *: MUTCD:1

The rationale and supporting evidence for these treatments can be found beginning on page 215 of this Handbook.

19. Signal Head Visibility

Place all required signal heads overhead and centered over each lane instead of placing them on pedestal poles (see Figure 35). Supplemental signal heads may be placed on pedestal posts as needed. Do not place a signal head that displays a circular green indication over or directly in front of a left-turn lane. Instead, place a shared signal face over the lane line between the left-turn and through lane, or slightly to the right of it, or place a separate signal face over the center of the left turn lane that uses a flashing yellow arrow as the permissive indication.

References *: MUTCD: 1

The rationale and supporting evidence for these treatments can be found beginning on page 216 of this Handbook.

20. High-Visibility Crosswalks

To allow drivers to more easily see pedestrians in a marked crosswalk, high-visibility crosswalk marking patterns should be utilized. Two examples of such markings include white diagonal lines at a 45 degree angle to the crosswalk or the "ladder" crosswalk design shown in Figure 36.

References *: MUTCD: 1

The rationale and supporting evidence for these treatments can be found beginning on page 216 of this Handbook.

21. Supplemental Pavement Marking for Stop and Yield Signs

Utilize "STOP AHEAD" word pavement markings to supplement stop ahead signs (W3-1) and "YIELD AHEAD" word pavement markings or yield ahead triangle symbol pavement markings to supplement yield ahead signs (W3-2). See Section 3B.20 of the 2009 MUTCD and Figure 37.

References *: MUTCD: 1

The rationale and supporting evidence for these treatments can be found beginning on page 217 of this Handbook.

22. Reduced Left-Turn-Conflict Intersections

A class of innovative intersection designs accommodate left-turns in unique ways, which greatly reduce, if not eliminate, unprotected left-turns at the intersection. Designs such as the median U-turn intersection (see Figure 38) and restrictedcrossing U-turn (RCUT) intersection (see Figure 39) have features that minimize the operational delay and potential for crashes due to left turns. Other intersection designs that are increasingly common include the displaced left-turn (DLT) intersection and the diverging diamond interchange (DDI). These innovative intersection designs should be considered for suitability during the engineering study for new and reconstructed intersections.

The rationale and supporting evidence for these treatments can be found beginning on page 218 of this Handbook.

23. Accessible Pedestrian Signal (APS) Treatments

A. Pushbutton-Activated Extended Pedestrian Crossing Phase

At crosswalks frequently used by aging pedestrians, consider inclusion of pushbutton-activated extension of the pedestrian crossing phase, using the required signage described by the MUTCD, as shown in Figure 40.

B. Passive Pedestrian Detection

Use of passive pedestrian detection to help aging pedestrians who have difficulty using the pushbutton or to detect pedestrians within the crosswalk that may need more time to complete the crossing maneuver. Passive pedestrian detection uses sensors to detect the presence of pedestrians and register a pedestrian call with the signal system; as a result, the pedestrian does not have to push a button to request a WALK signal or extended crossing time.

The rationale and supporting evidence for these treatments can be found beginning on page 219 of this Handbook.

24. Flashing Yellow Arrow

The flashing yellow arrow (see Figure 41) is the recommended signal indication for permissive left-turn movements at signalized intersections.

The rationale and supporting evidence for these treatments begins on page 220 of the Handbook.

References Legend

See pages 3 and 4 for full description of codes and acronyms of cited design guides.

• 1: most conservative
• 2: preferred among differing guides
• 3: new application of current practice
• 4: more specific, detailed or stringent
• 5: permissible only in accordance with MUTCD section 1A.10, Interpretations, Experimentations, and Changes