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Publication Number: FHWA-HRT-04-091
Date: August 2004

Signalized Intersections: Informational Guide

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CHAPTER 3 — GEOMETRIC DESIGN

TABLE OF CONTENTS

3.0 GEOMETRIC DESIGN

3.1 Channelization

3.2 Number of Intersection Legs

3.3 Intersection angle

3.4 Horizontal and Vertical Alignment

3.5 Corner Radius and Curb Ramp Design

3.5.1 Corner Radius

3.5.2 Curb Ramp Design

3.5.3 Detectable Warnings

3.6 Sight Distance

3.6.1 Stopping Sight Distance

3.6.2 Decision Sight Distance

3.6.3 Intersection Sight Distance

3.7  Pedestrian Facilities

3.8  Bicycle Facilities

 

LIST OF FIGURES

10
The photograph shows a raised median that restricts left-turn egress movements from a  driveway located between two signalized intersections
11
Pavement markings can be used to delineate travel lanes within wide intersections as  shown in the photograph
12  
Various right-turn treatments may be used, depending on the speed environment
13

Providing a dedicated left-turn lane reduces potential collisions between left-turning and through vehicles, increasing the capacity of the approach for both left and through traffic

14
The photo shows how double left-turn and double right-turn lanes can be used to  accommodate high-priority movements
15
Intersection skews increase both the intersection width and pedestrian crossing distance
16 
The photograph illustrates a multileg intersection
17
Potential conflicts at intersections with three and four legs
18
Curb ramp components
19
Examples of preferred designs
20
Examples of acceptable curb ramp designs
21
Examples of inaccessible designs
22
This crosswalk design incorporates the use of detectable warning surfaces into the curb ramps to facilitate navigation by a visually impaired pedestrian

     

LIST OF TABLES

Summary of best practices for curb ramp design and associated rationale
10
Requirements for detectable warning surfaces
11
Design values for stopping sight distance
12
Design values for decision sight distance for selected avoidance maneuvers

3.0   Geometric Design

This chapter presents geometric design guidelines for signalized intersections based on a review of technical literature and current design policy in the United States.

Geometric design of a signalized intersection involves the functional layout of travel lanes, curb ramps, crosswalks, bike lanes, and transit stops in both the horizontal and vertical dimensions. Geometric design has a profound influence on roadway safety; it shapes road user expectations and defines how to proceed through an intersection where many conflicts exist.

In addition to safety, geometric design influences the operational performance for all road users. Minimizing impedances, eliminating the need for lane changes and merge maneuvers, and minimizing the required distance to traverse an intersection all help improve the operational efficiency of an intersection.

The needs of all possible road users (see chapter 2) must be considered to achieve optimal safety and operational levels at an intersection. At times, design objectives may conflict between road user groups; the practitioner must carefully examine the needs of each user, identify the tradeoffs associated with each element of geometric design, and make decisions with all road user groups in mind.

This chapter addresses the following topics:

  • Principles of channelization.

  •   Number of intersection approaches.

  • Intersection angle.

  • Horizontal and vertical alignment.

  • Corner radius and curb ramp design

  • Detectable warnings.

  • Access control.

  • Sight distance.

  • Pedestrian facilities.

  • Bicycle facilities.

3.1 Channelization

a primary goal of intersection design is to limit or reduce the severity of potential road user conflicts. Basic principles of intersection channelization that can be applied to reduce conflicts are described below.(41)

1. Discourage undesirable movements. Designers can utilize corner radii, raised medians, or traffic islands to prevent undesirable or wrong-way movements. Examples include:

  • Preventing left turns from driveways or minor streets based on safety or operational concerns.

  • Designing channelization to prevent wrong way movements onto freeway ramps, one-way streets, or divided roadways.

  • Designing approach alignment to discourage undesirable movements.

Figure 10 shows how a raised median can be used to restrict undesirable turn movements within the influence of signalized intersections.

The picture shows a raised median on a six-lane divided arterial. A driveway into a large shopping center is located approximately 80 to 100 meters (240 to 300 feet) from signalized intersections on each side. The driveway access is restricted by the median to right-in/right-out/left-in.
Figure 10. The photograph shows a raised median that restricts left-turn egress movements from a driveway located between two signalized intersections.

2. Define desirable paths for vehicles. The approach alignment to an intersection as well as the intersection itself should present the roadway user with a clear definition of the proper vehicle path. This is especially important at locations with “unusual” geometry or traffic patterns such as highly skewed intersections, multileg intersections, offset-t intersections and intersections with very high turn volumes. Clear definition of vehicle paths can minimize lane changing and avoid “trapping” vehicles in the incorrect lane. Avoiding these undesirable effects can improve both the safety and capacity at an intersection.  Figure 11 shows how pavement markings can be applied to delineate travel paths.

The picture shows a wide four-way signalized intersection with double left-turn lanes on all approaches. Dotted pavement lines are used to guide left-turning vehicles into the appropriate lanes.
Figure 11. Pavement markings can be used to delineate travel lanes within wide intersections as shown in the photograph.

3. Encourage safe speeds through design. An effective intersection design promotes desirable speeds to optimize intersection safety. The appropriate speed will vary based on the use, type, and location of the intersection. On high-speed roadways with no pedestrians, it may be desirable to promote higher speeds for turning vehicles to remove turning vehicles from the through traffic stream as quickly and safely as possible. This can be accomplished with longer, smooth tapers and larger curb radii. On low-speed roadways or in areas with pedestrians, promotion of lower turning speeds is appropriate. This can be accomplished with smaller turning radii, narrower lanes, and/or channelization features. These are illustrated in figure 12.

The diagram illustrates (A) how channelization islands and larger curb radii accommodate higher speed right-turn movements and (B) how smaller curb radii can accommodate lower speed right-turn movements.
(a) Higher speed design

 

The diagram illustrates (A) how channelization islands and larger curb radii accommodate higher speed right-turn movements and (B) how smaller curb radii can accommodate lower speed right-turn movements.
(b) Lower speed design

Figure 12. Various right-turn treatments may be used,

depending on the speed environment.

4. Separate points of conflict where possible. Separation of conflict points can ease the driving task while improving both the capacity and safety at an intersection. The use of exclusive turn lanes, channelized right turns, and raised medians as part of an access control strategy are all effective ways to separate vehicle conflicts. Figure 13 illustrates how the addition of a left-turn lane can reduce conflicts with through vehicles traveling in the same direction.

The diagram shows a four-lane, four-way intersection. In the top example (A), the major street has two lanes in each direction, with left-turning vehicles sharing the lane with through vehicles. Through vehicles stack up behind a vehicle waiting to turn left. In the bottom example (B), a dedicated left-turn lane allows a left-turning vehicle to wait without impeding through traffic on the adjacent through lanes.
(a) Major street with shared left-through lane causes through vehicles to queue behind left-turning vehicles.

 

The diagram shows a four-lane, four-way intersection. In the top example (A), the major street has two lanes in each direction, with left-turning vehicles sharing the lane with through vehicles. Through vehicles stack up behind a vehicle waiting to turn left. In the bottom example (B), a dedicated left-turn lane allows a left-turning vehicle to wait without impeding through traffic on the adjacent through lanes.
(b) Major street with dedicated left-turn lane removes left-turning vehicles from the paths of through vehicles.

Figure 13. Providing a dedicated left-turn lane reduces

potential collisions between left-turning and through

vehicles, increasing the capacity of the approach for

both left and through traffic.

5. Facilitate the movement of high-priority traffic flows. Accommodating high-priority movements at intersections addresses both driver’s expectations and intersection capacity. The highest volume movements at an intersection typically define the intersection’s high-priority movements, although route designations and functional classification of intersecting roadways may also be considered. In low-density suburban and rural areas, it may be appropriate to give priority to motor vehicle movements; however, in some urban locations, pedestrians and bicyclists at times may be the highest priority users of the road system. Figure 14 shows an intersection where double left and right turn lanes are used to facilitate high-volume turning movements.

The picture shows a T-intersection with crosswalks and double left- and right-hand turn lanes that accommodate high-volume turning movements.
Photograph Credit and Copyright: www.portlandmaps.com, 2004
Figure 14. The photo shows how double left-turn and double right-turn lanes can be used to accommodate high-priority movements.

6. Design approaches to intersect at near right angles and merge at flat angles. Roadway alignments that cross as close to 90 degrees as practical can minimize the exposure of vehicles to potential conflicts and reduce the severity of a conflict. Skewed crossings produce awkward sight angles for drivers, which can be especially difficult for older drivers. Skewed crossings also result in additional distance for vehicles to traverse the intersections. This additional distance should be considered when developing the timing for a signal, as it may require the need for additional all-red clearance time.  Figure 15 shows how a skewed intersection approach can increase the distance to clear the intersection for pedestrians and vehicles.

Figure 15a. An intersection with a 90-degree angle between approaches, resulting in a crosswalk length of 18.6 meters (61 feet) and distance across the intersection of 31.1 meters (102 feet) from the leading edge of a crosswalk on the near side of the intersection to the trailing edge of a crosswalk on the far side of the intersection
(a) Intersection skew at 90 degrees.
Figure 15b. Intersections with skewed angle of 75. This results in increased crosswalk lengths of 19.2 meters (63 feet) and 23.2 meters (76 feet), respectively, and increased distance across the intersection of 32.9 meters (108 feet) and 36.6 meters (120 feet), respectively
(b) Intersection skew at 75 degrees.
Figure 15c. Intersection with skewed angle of 60 degrees. This results in increased crosswalk lengths of 19.2 meters (63 feet) and 23.2 meters (76 feet), respectively, and increased distance across the intersection of 32.9 meters (108 feet) and 36.6 meters (120 feet), respectively
(c) Intersection skew at 60 degrees.

Figure 15. Intersection skew increases both the intersection

width and pedestrian crossing distance.

7. Facilitate the desired scheme of traffic control. The design of a signalized intersection should attempt to maximize traffic safety and operations while providing operational flexibility. Lane arrangements, location of channelization islands, and medians should be established to facilitate pedestrian access and the placement of signs, signals, and markings. Consideration of these “downstream” issues as part of design can optimize the operation of an intersection. Providing exclusive left-turn bays that can accommodate left-turn movements can improve operations and safety while providing flexibility to accommodate varying traffic patterns. Positive offset left-turn lanes can improve sight distance for left-turning movements but may prohibit U-turns if insufficient width is available. Reversible lanes may be appropriate for arterials that experience heavy directional peaks in traffic volumes during commuter periods.

8. Accommodate decelerating, slow, or stopped vehicles outside higher speed through traffic lanes. Speed differentials between vehicles in the traffic stream are a primary cause of traffic crashes. Speed differentials at intersections are inherent as vehicles decelerate to facilitate a turning maneuver. The provision of exclusive left- and right-turn lanes can improve safety by removing slower moving turning vehicles from the higher speed through traffic stream and reducing potential rear-end conflicts. In addition, through movements will experience lower delay and fewer queues.

9. Provide safe refuge and wayfinding for bicyclists and pedestrians. Intersection design must consider the needs of roadway users other than motorists. Intersection channelization can provide refuge and/or reduce the exposure distance for pedestrians and bicyclists within an intersection without limiting vehicle movement. The use of raised medians, traffic islands, and other pedestrian-friendly treatments should be considered as part of the design process. Wayfinding may also be an issue, particularly at intersections with complicated configurations.

3.2 Number of Intersection Legs

While the geometry of various types of intersections may vary, the complexity of an intersection increases with an increasing number of approach legs to the intersections, as shown in figures 16 and 17. The latter shows the number and type of conflicts that occur at intersections with three and four legs, respectively. The number of potential conflicts for all users increases substantially at intersections with more than four legs. Note that many potential conflicts, including crossing and merging conflicts, can be managed (but not eliminated) at a signalized intersection by separating conflicts in time.

The aerial picture shows a busy multileg intersection with five roads converging and seven crosswalks of varying lengths.
Figure 16. The photograph illustrates a multileg intersection.
The three-leg intersection shows nine potential conflicts: three diverging, three merging, and three crossing the intersection. The four-leg intersection shows 32 total potential conflicts: 8 diverging, 8 merging, and 16 crossing the intersection.
(a) Three-leg intersection.
The three-leg intersection shows nine potential conflicts: three diverging, three merging, and three crossing the intersection. The four-leg intersection shows 32 total potential conflicts: 8 diverging, 8 merging, and 16 crossing the intersection.
(b) Four-leg intersection.

Figure 17. Potential conflicts at intersections with three and four legs.

3.3 Intersection angle

The angle of intersection of two roadways can influence both the safety and operational characteristics of an intersection. Heavily skewed intersections not only affect the nature of conflicts, but they produce larger, open pavement areas that can be difficult for drivers to navigate and pedestrians to cross. Such large intersections can also be more costly to build and maintain.

Undesirable operational and safety characteristics of skewed intersections include:

  • Difficulty in accommodating large vehicle turns. Additional pavement, channelization, and right-of-way may be required. The increase in pavement area poses potential drainage problems and gives smaller vehicles more opportunity to “wander” from the proper path.

  • Vehicles crossing the intersection are more exposed to conflicts. This requires longer clearance intervals and increased lost time, which reduces the capacity of the intersection.

  • Pedestrians and bicyclists are exposed to vehicular traffic longer. Longer pedestrian intervals may be required, which may have a negative impact on the intersection’s capacity.

  • Pedestrians with visual disabilities may have difficulty finding their way to the other side of the street when crossing.

  • Driver confusion may result at skewed crossings. Woodson, Tillman, and tillman found that drivers are more positive in their sense of direction when roadways are at right angles to each other.(42) Conversely, drivers become more confused as they traverse curved or angled streets.

Skewed intersections are generally related to right-angle type crashes that can be associated with poor sight distance. AASHTO policy and many State design standards permit skewed intersections of up to 60 degrees.(3) Gattis and Low conducted research to identify constraints on the angle of a left-skewed intersection as it is affected by the vehicle body’s limiting a driver line-of-sight to the right.(43) Their findings suggest that if roadway engineers are to consider the limitations created by vehicle design, a minimum intersection angle of 70 to 75 degrees will offer an improved line of sight. FHWA’s Highway Design Handbook for Older Drivers and Pedestrians recommends intersection angles of 90 degrees for new intersections where right-of-way is not a constraint, and angles of not less than 75 degrees for new facilities or redesigns of existing facilities where right-of-way is restricted.(12)

3.4 Horizontal and Vertical Alignment

The approach to a signalized intersection should promote awareness of an intersection by providing the required stopping sight distance in advance of the intersection. This area is critical as the approaching driver or bicyclist begins to focus on the tasks associated with navigating the intersection.

To meet the driver’s or cyclist’s expectations on approaches to an intersection, the following guidelines are suggested:

  • Avoid approach grades to an intersection of greater than 6 percent. On higher design speed facilities (80 km/h (50 mph) and greater), a maximum grade of 3 percent should be considered.

  • Avoid locating intersections along a horizontal curve of the intersecting road.

  • Strive for an intersection platform (including sidewalks) with cross slope not exceeding 2 percent, as needed for accessibility.

3.5 Corner Radius and Curb Ramp Design

Intersection corners that are designed appropriately accommodate all users. The selection of corner radius and curb ramp design should be guided by pedestrian crossing and design vehicle needs at the intersection. In general, it is recommended to provide a pedestrian crossing that is as near to perpendicular to the flow of traffic as practical with no intermediate angle points. This keeps pedestrian crossing time and exposure to a minimum, which may allow more efficient operation of the signal. It also aids visually impaired pedestrians in their wayfinding task by eliminating changes in direction that may not be detectable.

Corner radii should also be designed to accommodate the turning path of a design vehicle to avoid encroachment on pedestrian facilities and opposing lanes of travel.

3.5.1 Corner Radius

The corner radii of an intersection should be designed to facilitate the turning and tracking requirements of the selected design vehicle. Other considerations when designing a corner radius include location of traffic control devices (signal poles, controller, signs, etc.), the need to provide channelizing islands, and available right-of-way. The corner radii should be compatible with other intersection features and the speed environment. For example, larger radii are more compatible with high-speed facilities with few pedestrians, whereas smaller radii are more compatible with low-speed facilities with many pedestrians.(41)

Factors that influence the selection of appropriate corner radii include the following:

  • Design vehicle. Selection of a design vehicle should be based on the largest vehicle type that will regularly use an intersection. Often, a design vehicle is mandated by agency policy, regardless of vehicle mix. In certain instances, more than one design vehicle may be appropriate depending on traffic patterns.

  • Angle of intersection. Large intersection skew angles make turning maneuvers more difficult, particularly for larger vehicles. This has the potential to increase the overall size of the intersection, making drainage difficult and increasing signal clearance intervals to clear the intersection.

  • Pedestrians and bicyclists. In areas of high pedestrian and bike use, smaller radii are desirable to reduce turning speeds and decrease the distance for pedestrians and bikes to cross the street.

  • Constraints. Multicentered curves or simple curves with tangent offsets can be used to better match the turn path of the design vehicle and reduce required right-of-way.

3.5.2 Curb Ramp Design

Curb ramps provide access for people who use wheelchairs and scooters. Curb ramps also aid people with strollers, luggage, bicycles, and other wheeled objects in negotiating the intersection. The basic components of a curb ramp, including ramp, landing, detectable warning, flare, and approach, are diagrammed in figure 18. The ADAAG require that curb ramps be provided wherever an accessible route crosses a curb, which includes all designated crosswalks at new and retrofitted signalized intersections.(33) While curb ramps increase access for mobility-impaired pedestrians, they can decrease access for visually impaired pedestrians by removing the vertical curb face that provides an important tactile cue. This tactile cue is instead provided by a detectable warning surface placed at the bottom of the ramp, which provides information on the boundary between the sidewalk and roadway.

The diagram shows a flared ramp in a sidewalk with labeled components. The ramp consists of a central ramp area, detectable warnings on the ramp next to the gutter, and angled flares on each side. The sidewalk consists of a landing area behind the ramp and approaches on each side.
Figure 18. Curb ramp components.

Table 9, adapted from FHWA’s Designing Sidewalks and trails for Access, Part 2: Best Practices Design Guide, provides a summary of recommended fundamental practices for curb ramp design, along with the rationale behind each practice.(34) A designer can apply these principles in designing intersections in a wide variety of circumstances.

Figures 19-21 provides examples of three categories of typical curb ramp treatments used at signalized intersections: those that should be implemented wherever possible (“preferred designs”), those that meet minimum accessibility requirements but are not as effective as the preferred treatments (“acceptable designs”), and those that are inaccessible and therefore should not be used in new or retrofit designs (“inaccessible designs”). Additional guidance and design details can be found in the source document.(34)

Table 9. Summary of best practices for curb ramp design and associated rationale.

Best Practice

Rationale

Provide a level maneuvering area or landing at the top of the curb ramp.

Landings are critical to allow wheelchair users space to maneuver on or off the ramp. Furthermore, people who are continuing on the sidewalk will not have to negotiate a surface with a changing grade or cross slope.

Clearly identify the boundary between the bottom of the curb ramp and the street with a detectable warning.

Without a detectable warning, people with visual impairments may not be able to identify the boundary between the sidewalk and the street. (Note that detectable warnings are a requirement of ADA as of July 2001.)

Design ramp grades that are perpendicular to the curb.

Assistive devices for mobility are unusable if one side of the device is lower than the other or if the full base of support (e.g., all four wheels on a wheelchair) is not in contact with the surface. This commonly occurs when the bottom of a curb ramp is not perpendicular to the curb.

Place the curb ramp within the marked crosswalk area.

Pedestrians outside of the marked crosswalk are less likely to be seen by drivers because they are not in an expected location.

Avoid changes of grade that exceed 11 percent over a 610 mm (24 inch) interval.

Severe or sudden grade changes may not provide sufficient clearance for the frame of the wheelchair, causing the user to tip forward or backward.

Design the ramp so that it does not require turning or maneuvering on the ramp surface.

Maneuvering on a steep grade can be very hazardous for people with mobility impairments.

Provide a curb ramp grade that can be easily distinguished from surrounding terrain; otherwise, use detectable warnings.

Gradual slopes make it difficult for people with visual impairments to detect the presence of a curb ramp.

Design the ramp with a grade of 7.1 ±1.2 percent. Do not exceed 8.33 percent (1:12).

Shallow grades are difficult for people with vision impairments to detect, but steep grades are difficult for those using assistive devices for mobility.

Design the ramp and gutter with a cross slope of 2.0 percent.

Ramps should have minimal cross slope so users do not have to negotiate a steep grade and cross slope simultaneously.

Provide adequate drainage to prevent the accumulation of water or debris on or at the bottom of the ramp.

Water, ice, or debris accumulation will decrease the slip resistance of the curb ramp surface.

Provide transitions from ramps to gutter and streets that are flush and free of level changes.

Maneuvering over any vertical rise such as lips and defects can cause wheelchair users to propel forward when wheels hit this barrier.

Align the curb ramp with the crosswalk so there is a straight path of travel from the top of the ramp to the center of the roadway to the curb ramp on the other side.

Where curb ramps can be seen in advance, people using wheelchairs often build up momentum in the crosswalk in order to get up the curb ramp grade (i.e., they “take a run at it”). This alignment may be useful for people with vision impairments.

Provide clearly defined and easily identified edges or transitions on both sides of the ramp to contrast with the sidewalk.

Clearly defined edges assist users with vision impairments to identify the presence of the ramp when it is approached from the side.

Source: Adapted from reference 34, table 7-1.

(A) Perpendicular curb ramps with flares and a level landing.
(a)
(B) Perpendicular curb ramps with returned curbs and a level landing.
(b)
(C) Two parallel curb ramps with a lowered curb between the ramps.
(c)
(D) Two parallel curb ramps with a lowered curb between the ramps
(d)
(E) Two combination curb ramps on a corner with a wide turning radius.
(e)
(F) A curb extension with two perpendicular curb ramps with returned curbs and level landings.
(f)
  1. Perpendicular curb ramps with flares and a level landing.
  2. Perpendicular curb ramps with returned curbs and a level landing.
  3. Two parallel curb ramps on a wide turning radius.
  4. Two parallel curb ramps with a lowered curb.
  5. Two combination curb ramps on a corner with a wide turning radius.
  6. A curb extension with two perpendicular curb ramps with returned curbs and level landings.
Figure 19.  Examples of preferred designs.

(A) Perpendicular curb ramps design perpendicular to the curb on a corner with a wide turning radius.
(a)
(B) Diagonal curb ramp with flares and a level landing, in addition to at least 1.22 meters (48 inches) of clear space.
(b)
(C) Diagonal curb ramp with returned curbs, a level landing, and sufficient clear space in the crosswalk.
(c)
(D) Single parallel curb ramp with at least 1.22 meters (48 inches) clear space.
(d)
(E) Two built-up curb ramps.
(e)
(F) Partially built-up curb ramps.
(f)
  1. Perpendicular curb ramps, oriented perpendicular to the curb, on a corner with a wide turning radius.
  2. Diagonal curb ramp with flares and a level landing, in addition to at least 1.22 m (48 inch) of clear space.
  3. Diagonal curb ramp with returned curbs, a level landing, and sufficient clear space in the crosswalk.
  4. Single parallel curb ramp with at least 1.22 m (48 inch) clear space.
  5. Two built-up curb ramps.
  6. Partially built-up curb ramps.

Figure 20.  Examples of acceptable curb ramp designs.

 

(A) Perpendicular curb ramps without a landing.
(a)
(B) On a corner with a wide turning radius, curb ramps are aligned parallel with the crosswalk.
(b)
(C) Diagonal curb ramp with no clear space or no level area at the bottom of the curb ramp.
(c)
(D) Diagonal curb ramps without a level landing.
(d)
  1. Perpendicular curb ramps without a landing.
  2. On a corner with a wide turning radius, curb ramps are aligned parallel with the crosswalk.
  3. Diagonal curb ramp with no clear space or no level area at the bottom of the curb ramp.
  4. Diagonal curb ramps without a level landing.
Figure 21.  Examples of inaccessible designs.

Source: Reproduced from reference 34, table 7-2

 

3.5.3 Detectable Warnings

The ADAAG require that a detectable warning surface be applied to the surface of the curb ramps and within the refuge of any medians and islands (defined in the ADAAG as “hazardous vehicle areas”) to provide tactile cues to individuals with visual impairments.(33) Detectible warnings consist of a surface of truncated domes built in or applied to walking surfaces; the domes provide a distinctive surface detectable by cane or underfoot. This surface alerts visually impaired pedestrians of the presence of the vehicular travel way, and provides physical cues to assist pedestrians in detecting the boundary from sidewalk to street where curb ramps and blended transitions are devoid of other tactile cues typically provided by a curb face.

At the face of a curb ramp and within the refuge area of any median island, a detectable warning surface should be applied as shown in figure 22. The detectable warning surface begins at the curb line and extends into the ramp or pedestrian refuge area a distance of 610 mm (24 inches). For a median island, this creates a minimum clear space of 610 mm (24 inches) between the detectable warning surfaces for a minimum median island width of 1.8 m (6 ft) at the pedestrian crossing. This is a deviation from the requirements of the ADAAG (§4.29.5), which requires a surface width of 915 mm (36 inches). However, this deviation is necessary to enable visually impaired pedestrians to distinguish where the refuge begins and ends from the adjacent roadway where the minimum 1.8 m (6 ft) refuge width is provided.

Table 10 summarizes ADAAG requirements for detectable warning surfaces.

The picture shows a visually impaired person using a cane stepping onto the detectable warning surface of a ramp, about to enter the street. Photograph Credit: Lee Rodegerdts, 2003
Figure 22.This crosswalk design incorporates the use of detectable warning surfaces into the curb ramps to facilitate navigation by a visually impaired pedestrian.

 

Table 10. Requirements for detectable warning surfaces.

Legislation

Americans with Disabilities act accessibility Guidelines(33)

Draft Guidelines on
Accessible Public Rights-of-Way
(44)

Applicability

Required under existing regulations.

These guidelines are in the rulemaking process and are therefore not enforceable. They will be incorporated into the ADAAG; however, the recommendations listed below are subject to revision prior to the issuance of a final rule.

Type

Raised truncated domes.

Raised truncated domes aligned in a square grid pattern.

Dome size

Nominal diameter: 23 mm (0.9 inches).
Nominal height: 5 mm (0.2 inches).

Base diameter: 23 mm (0.9 inches) minimum, 36 mm (1.4 inches) maximum.
Ratio of top diameter to base diameter: 50% minimum, 65% maximum.
Height: 5 mm (0.2 inches).

Dome spacing

Nominal center-to-center spacing: 60 mm (2.35 inches).

Center-to-center spacing: 41 mm (1.6 inches) minimum, 61 mm (2.4 inches) maximum.
Base-to-base spacing: 16 mm (0.65 inches) minimum, measured between the most adjacent domes on square grid.

Contrast

Detectable warning surfaces must contrast visually with adjacent walking surfaces either light-on-dark, or dark-on-light.
The material used to provide contrast must be an integral part of the walking surface.

Detectable warning surfaces must contrast visually with adjacent walking surfaces either light-on-dark, or dark-on-light.

Size

At curb ramps: The detectable warning must extend the full width and depth of the curb ramp.

At curb ramps, landings, or blended transitions connecting to a crosswalk: Detectable warning surfaces must extend 610 mm (24 inches) minimum in the direction of travel and the full width of the curb ramp, landing, or blended transition. The detectable warning surface must be located so that the edge nearest the curb line is 150 mm (6 inches) minimum and 205 mm (8 inches) maximum from the curb line.

Within median islands, the boundary between the curbs must be defined by a continuous detectable warning 915 mm (36 inches) wide, beginning at the curb line.

Within median islands, the detectable warning surface must begin at the curb line and extend into the pedestrian refuge a minimum of 610 mm (24 inches). Detectable warnings must be separated by a minimum length of walkway of 610 mm (24 inches) without detectable warnings.

The Draft Guidelines on accessible Public Rights-of-Way, developed by the U.S. Access board, issued a similar recommendation for use of a 610-mm (24-inch) width for detectable warning surfaces.(44) This is consistent with the existing ADAAG requirements for truncated dome detectable warning surfaces at transit platforms. The draft public right-of-way guidelines are based upon the recommendations of the Public Rights of Way Access advisory Committee as published in the report Building a True Community.(45) For detectable warning surfaces, both the U.S. Access board and FHWA are encouraging the use of the new (recommended) design pattern and application over the original ADAAG requirements.(33)

3.6 Sight Distance

A driver’s ability to see the road ahead and other intersection users is critical to safe and efficient use of all roadway facilities, especially signalized intersections. Stopping sight distance, decision sight distance, and intersection sight distance are particularly important at signalized intersections.

3.6.1 Stopping Sight Distance

Stopping sight distance is the distance along a roadway required for a driver to perceive and react to an object in the roadway and to brake to a complete stop before reaching that object. Stopping sight distance should be provided throughout the intersection and on each entering and exiting approach. Table 11 gives recommended stopping sight distances for design, as computed from the equations provided in the AASHTO policy.(3)

Table 11. Design values for stopping sight distance.

Speed
(km/h)

Computed Distance*
(m)

Design Distance
(m)

Speed
(mph)

Computed Distance*
(ft)

Design Distance
(ft)

  20

  18.5

  20

 

15

  76.7

  80

  30

  31.2

  35

 

20

111.9

115

  40

  46.2

  50

 

25

151.9

155

  50

  63.5

  65

 

30

196.7

200

  60

  83.0

  85

 

35

246.2

250

  70

104.9

105

 

40

300.6

305

  80

129.0

130

 

45

359.8

360

  90

155.5

160

 

50

423.8

425

100

184.2

185

 

55

492.4

495

110

215.3

220

 

60

566.0

570

120

248.6

250

 

65

644.4

645

* Assumes 2.5 s perception-braking time, 3.4 m/s2 (11.2 ft/s2) driver deceleration

Source: Reference 3, exhibit 3-1.

Stopping sight distance should be measured using an assumed height of driver’s eye of 1,080 mm (3.5 ft) and an assumed height of object of 600 mm (2.0 ft).(3)

3.6.2 Decision Sight Distance

Decision sight distance is “the distance needed for a driver to detect an unexpected or otherwise difficult-to-perceive information source or condition in a roadway environment that may be visually cluttered, recognize the condition or its potential threat, select an appropriate speed and path, and initiate and complete the maneuver safely and efficiently.”(3, p. 115) Decision sight distance at intersections is applicable for situations where vehicles must maneuver into a particular lane in advance of the intersection (e.g., alternative intersection designs using indirect left turns).

Decision sight distance varies depending on whether the driver is to come to a complete stop or make some kind of speed, path, or direction change. Decision sight distance also varies depending on the environment—urban, suburban, or rural. Table 12 gives recommended values for decision sight distance, as computed from equations in the AASHTO policy.(3)

Table 12. Design values for decision sight distance for selected avoidance maneuvers.

Metric (m)

Speed
(km/h)

A

B

C

D

E

  50

70

155

145

170

195

  60

95

195

170

205

235

  70

115

235

200

235

275

  80

140

280

230

270

315

  90

170

325

270

315

360

100

200

370

315

355

400

110

235

420

330

380

430

120

265

470

360

415

470

 

U.S. Customary (ft)

Speed
(mph)

A

B

C

D

E

30

220

490

450

535

620

35

275

590

525

625

720

40

330

690

600

715

825

45

395

800

675

800

930

50

465

910

750

890

1030

55

535

1030

865

980

1135

60

610

1150

990

1125

1280

65

695

1275

1050

1220

1365

 

Avoidance Maneuver A: Stop on rural road, time (t) = 3.0 s.

Avoidance Maneuver B: Stop on urban road, t = 9.1 s.

Avoidance Maneuver C: Speed/path/direction change on rural road, t = 10.2 s to 11.2 s.

Avoidance Maneuver D: Speed/path/direction change on suburban road, t = 12.1 s to 12.9 s.

Avoidance Maneuver E: Speed/path/direction change on urban road, t = 14.0 s to 14.5 s.

Source: Reference 3, exhibit 3-3.

3.6.3 Intersection Sight Distance

Intersection sight distance is the distance required for a driver without the right of way to perceive and react to the presence of conflicting vehicles and pedestrians.

Intersection sight distance is traditionally measured through the determination of a sight triangle. This triangle is bounded by a length of roadway defining a limit away from the intersection on each of the two conflicting approaches and by a line connecting those two limits. Intersection sight distance should be measured using an assumed height of driver’s eye of 1,080 mm (3.5 ft) and an assumed height of object of 1,080 mm (3.5 ft).(3) The area within the triangle is referred to as the clear zone and should remain free from obstacles.

The reader is encouraged to refer to the AASHTO policy, pp. 654-680, for a complete discussion of intersection sight distance requirements.(3) Intersection sight distance at signalized intersections is generally simpler than for stop-controlled intersections. The following criteria should be met:

  • The first vehicle stopped on an approach should be visible to the first driver stopped on each of the other approaches.

  • Vehicles making permissive movements (e.g., permissive left turns, right turns on red, etc.) should have sufficient sight distance to select gaps in oncoming traffic.

  • Permissive left turns should satisfy the case for left turns from the major road (Case F, reference 3).

  • Right turns on red should satisfy the case for a stop-controlled right turn from the minor road (Case B2, reference 3).

For signalized intersections where two-way flashing operation is planned (i.e., flashing yellow on the major street and flashing red on the minor street), departure sight triangles for Case B should be provided for the minor-street approaches.(3)

3.7 Pedestrian Facilities

Pedestrian facilities should be provided at all intersections in urban and suburban areas. In general, design of the pedestrian facilities of an intersection with the most challenged users in mind—pedestrians with mobility or visual impairments should be done.  The resulting design will serve all pedestrians well. In addition, the ADA requires that new and altered facilities constructed by, on behalf of, or for the use of State and local government entities be designed and constructed to be readily accessible to and usable by individuals with disabilities.(33) Therefore, it is not only good practice to design for all pedestrian types, but it is also a legal requirement.

Pedestrians are faced with a number of disincentives to walking, including centers and services located far apart, physical barriers and interruptions along pedestrian routes, a perception that routes are unsafe due to motor vehicle conflicts and crime, and routes that are esthetically unpleasing.(46)

Key elements that affect a pedestrian facility that practitioners should incorporate into their design are listed below:(47)

  • Keep corners free of obstructions to provide enough room for pedestrians waiting to cross.

  • Maintain adequate lines of sight between drivers and pedestrians on the intersection corner and in the crosswalk.

  • Ensure curb ramps, transit stops (where applicable), pushbuttons, etc. are easily accessible and meet ADAAG design standards.

  • Clearly indicate the actions pedestrians are expected to take at crossing locations.

  • Design corner radii to ensure vehicles do not drive over the pedestrian area yet are able to maintain appropriate turning speeds.

  • Ensure crosswalks clearly indicate where crossings should occur and are in desirable locations.

  • Provide appropriate intervals for crossings and minimize wait time.

  • Limit exposure to conflicting traffic, and provide refuges where necessary.

  • Ensure the crosswalk is a direct continuation of the pedestrian's travel path.

  • Ensure the crossing is free of barriers, obstacles, and hazards.

3.8 Bicycle Facilities

Some intersections have on-street bicycle lanes or off-street bicycle paths entering the intersection. When this occurs, intersection design should accommodate the needs of cyclists in safely navigating such a large and often complicated intersection. Some geometric features that should be considered include:

  • Bike lanes and bike lane transitions between through lanes and right turn lanes.

  • Left turn bike lanes.

  • Median refuges with a width to accommodate a bicycle: 2.0 m (6 ft) = poor; 2.5 m (8 ft) = satisfactory; 3.0 m (10 ft) = good.(21, p. 52)

  • Separate facilities if no safe routes can be provided through the intersection itself.

The interaction between motor vehicles and bicyclists at interchanges with merge and diverge areas is especially complex, and some signalized intersections also have merge and diverge areas due to free right turns or diverted movements (see chapter 10). AASHTO recommends that “[i]f a bike lane or route must traverse an interchange area, these intersection or conflict points should be designed to limit the conflict areas or to eliminate unnecessary uncontrolled ramp connections to urban roadways.”(21, p. 62)

 

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