Road Management & Engineering Journal
Copyright © 1997 by TranSafety, Inc.
April 1, 1997|
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Making Intersections Safer for Pedestrians
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Making Intersections Safer for PedestriansExisting guidelines for highway engineers deal primarily with designing highways and streets for vehicles; little consideration is given to the needs of pedestrians. Martin T. Pietrucha of Pennsylvania State University's Transportation Institute and Kenneth S. Opiela of the Transportation Research Board addressed integrating the design concerns of these two very different modes of travel in their article "Safe Accommodation of Pedestrians at Intersections." The article appeared in Transportation Research Record No. 1385--Highway and Facility Design: Intersection and Interchange Design.
In writing this article, Pietrucha and Opiela carefully examined "the principal guidance for highway design decisions in the United States"--the American Association of State Highway and Transportation Officials' (AASHTO's) A Policy on the Geometric Design of Streets and Highways. This guide is commonly called the Green Book. The authors' goal was an examination of the Green Book's recommendations for roadway design within the context of pedestrian needs. They focused on the Green Book's guidelines for intersections (because competition for space between vehicles and pedestrians is most critical at intersections) and attempted to learn:
AASHTO published the Green Book in 1984 to update and consolidate manuals on the design of highway facilities. The revised 1990 edition recognizes pedestrian needs but, according to Pietrucha and Opiela, "does not adequately address all of the aspects related to providing safe and convenient conditions for the pedestrian." Therefore, their article critiqued the Green Book and made suggestions for changes and additions in two areas: (1) general design guidelines related to pedestrians and (2) design guidelines for at-grade intersections.
Critique of General Design Guidelines in the Green Book
Highway functions. To determine applicable design criteria, the Green Book establishes guidelines by categorizing roadways according to the function they serve in relation to other roadways. The roadway's function in serving the needs of pedestrians, however, does not enter the formula. The authors suggested that the six function classes defined for roadways should have subclasses to suggest how designs could integrate pedestrian needs. Design accommodations would vary depending on the needs of the pedestrian group served; e.g., lunchtime shoppers, children walking to school, or commuters waiting for public transportation. Designs would also vary with the speed and volume of traffic sharing the roadway with pedestrians.
Pedestrian Characteristics. Chapter II of the Green Book discusses simple models for designing roadways and intersections that consider the space required by pedestrians and the speed at which they walk. The Green Book provides information on accommodating physically, visually, and mentally challenged pedestrians. Emphasis, however, is on pedestrians in urban areas, to the exclusion of guidelines for suburban and rural roadways. Pietrucha and Opiela were especially concerned about pedestrians being in danger when they walk where motorists are not expecting them; e.g., along suburban roadways where rapid population growth has led to pedestrians and vehicles sharing high-speed roadways that have no shoulders or walkways. The authors felt Green Book descriptions of pedestrians might indirectly discourage consideration of their needs. They quoted the Green Book describing pedestrians as "unpredictable, obstinate, ignorant, inattentive, or defiant." Pietrucha and Opiela pointed out that pedestrians are also "unprotected, slow moving, and extremely fragile."
Physical Characteristics. Based on research by J.J. Fruin, the Green Book estimates the average pedestrian occupies an ellipse 24 inches by 18 inches. Pietrucha and Opiela mentioned that designs should also consider the additional space needs of older people with canes and pedestrians with shopping carts or baby carriages. While the Green Book recognizes 2.5 to 6.0 feet/second as the range of speeds at which pedestrians move and 4.0 feet/second as the average speed, some research suggests wider ranges and averages of from 3.0 to 4.5 feet/second. The needs of older pedestrians and the disabled would suggest allowing for slower speeds.
Disabled Pedestrian Characteristics. The Green Book mentions that curbs and barriers present difficulty to physically and visually challenged pedestrians. It does not, however, consider current research that shows those with difficulty walking might benefit by a higher level of walking-surface friction.
Walkways. Here the Green Book recommends more sidewalks and walkways in rural and suburban areas where there is heavy pedestrian traffic. Sidewalks should be four feet to eight feet wide and separated from the road by at least two feet of clearance. The Green Book includes a set of guidelines for sidewalk designs that keep pedestrians off the road and on continuous sidewalks. These designs meet the requirements of the elderly and disabled. Guidelines call for well-marked crosswalks, barrier curbs and full barriers on low-speed and high-speed roads respectively, and special treatment for bridge walkways. Pietrucha and Opiela felt these guidelines do not offer "adequate quantitative criteria nor full integration into the facility design process." They also mentioned there is no consideration of intersection design with an eye toward holding many waiting and moving pedestrians.
Crossings. Guidelines in the Green Book describe situations where the number of pedestrians and/or unusual hazards call for separate pedestrian crossings or such accommodations as overpasses or underpasses.
Critique of At-Grade Intersection Guidelines in the Green Book
The general pedestrian-facility guidelines described above come from the Green Book's first two chapters. Pietrucha and Opiela moved from these general guidelines to the Green Book's Chapter IX on at-grade intersections. Again, they broke their discussion into topics.
Intersection Types. The authors would like to see the Green Book include discussion of how each intersection type would serve the needs of pedestrians. This would encourage designers to consider pedestrian needs when designing intersections. The Green Book does not mention at-grade intersection treatments that would improve safety for pedestrians; e.g., flared (enlarged, rounded) sidewalk corners that give pedestrians shorter crossing distances, make them more visible to motorists, allow them an improved view of traffic, and increase the sidewalk's capacity to hold waiting groups of pedestrians.
Capacity Considerations. In its design recommendations, the Green Book uses a formula to figure the vehicular capacity of an intersection; however, it does not incorporate formulas that apply capacity criteria to pedestrians. Guidelines based on the number of pedestrians using an intersection and the physical characteristics of pedestrians could provide criteria for designing walkways, queuing areas, street corners, and crosswalks.
Alignment and Profile. While the Green Book emphasizes that intersections are points of conflict for vehicles and pedestrians, it does not provide guidelines for reducing conflict. Discussions of intersection alignment (90-degree angles versus non- perpendicular intersections) refer to vehicle needs but not to pedestrian needs. Hills surrounding intersections can also block sight lines for vehicles and pedestrians. Pietrucha and Opiela suggested three-dimensional views of intersections (rather than the two-dimensional views used) to help designers analyze sight-line problems.
Intersection Curves and Turning Radii. Where the radius of a curve accommodates vehicles of all lengths, there is a buffer zone needed to provide safety for pedestrians. The Green Book recommends additional right-of-way to make intersections at curves in urban areas safer for pedestrians. The authors suggested further guidelines for curved suburban and rural intersections. In addition, the angle of crosswalks at curves requires further clarification. Research suggests crosswalks at right angles to the curve would be safest.
Islands and Medians. Citing inadequate design details and criteria for medians and refuge islands in the Green Book, the authors wrote that islands and medians aid pedestrians because:
To help pedestrians cross wide streets or safely negotiate irregular, high-speed, or confusing intersections, the authors suggested guidelines that specify refuge islands for intersections wider than 75 feet. There should also be refuge islands if the intersection cannot be crossed during the time span of one green light by a pedestrian moving at 3.5 feet/second. These considerations are especially important where older pedestrians use the intersection. Refuge islands should be no less than four feet wide (six feet is preferred) and at least 12 feet long. Island designers also need to consider curb ramps for wheelchairs. Because medians and islands provide a stopping place so pedestrians can cross highways one lane at a time, shorter gaps in traffic are necessary for safe passage. In addition, pedestrians can concentrate on watching traffic from only one direction at a time. Designs should discourage pedestrians waiting to cross from walking parallel to traffic along the refuge island. Such behavior increases the number of possible conflict points.
Traffic Control Devices (TCDs). Designers should install TCDs (i.e., signals, signs, and pavement markings) according to guidelines that consider all aspects of the intersection and the needs of motorists and pedestrians. The authors discussed TCDs under three headings: Flow Control, Device Application, and Other Controls.
Channelization. The Green Book should rewrite its ten principles of channelization to reflect the needs of pedestrians and motorists.
Cross Section Design. Green Book recommendations for design of pavement slope, lane width, shoulders, curbs, and walkways on suburban and rural roads do not consider the needs of pedestrians. About 15 percent of pedestrian fatalities in suburban and rural areas result from pedestrians being hit while walking along the roadway; therefore, the authors felt pedestrians should be considered in each of these design areas.
The authors mentioned factors designers should address to meet pedestrian needs on suburban and rural roads. Severely sloped shoulders may cause pedestrians to move closer to traffic to avoid walking on the slope. Narrow lanes and shoulders create increased hazards as pedestrians walk close to high-speed traffic, and raised curbs present a barrier to some pedestrians. Knoblauch et. al. in their Federal Highway Administration (FHWA) report "Investigation of Exposure Based Pedestrian Accident Areas: Crosswalks, Sidewalks, Local Streets and Major Arterial" (Report FHA/RD- 88/038) offered guidelines on installing sidewalks or walkways along urban, suburban, and rural roads based on roadway function and development density. Pietrucha and Opiela suggested such guidelines in the Green Book would be helpful to highway designers.
Wheelchair Ramps at Intersections. Pietrucha and Opiela called the discussion of wheelchair ramps in the Green Book "woefully inadequate." They directed designers to the FHWA's Provisions for Elderly and Handicapped Pedestrians: Development of Priority Accessible Networks, An Implementation Manual (J.A. Templar, 1980) to find simple, effective designs.
Driveways and Access Management. Referring to Green Book coverage of driveways as "a serious deficiency," Pietrucha and Opiela called for increased attention to access control in guidelines for highway designers. Driveways create an intersection where they cross sidewalks. There are generally no warnings to alert pedestrians to the hazards of such intersections.
Pietrucha and Opiela concluded that the AASHTO Green Book, in particular Chapter IX on intersections, is vehicle-oriented and provides highway designers only limited guidance on pedestrian needs. They suggested updating the Green Book to weave pedestrian concerns into the text and to develop an integrated approach to highway design.
Pietrucha and Opiela recommended research in four areas:
Copyright © 1997 by TranSafety, Inc.