JVIB Copyright © 2009 American Foundation for the Blind. All rights reserved. File created April 10, 2009. This is not the final version of record. The
following article was published in the Journal of Visual Impairment &
Blindness, November 2001 • Volume 95 • Number 11. The final version of record can be found
Pedestrian Clearance Intervals at Modern Intersections:
Implications for the Safety of Pedestrians Who Are Visually Impaired
Janet M. Barlow, Lukas Franck, Billie Louise Bentzen, and Dona Sauerburger
The authors wish to thank Scott Wainwright, Federal Highway Administration, former chair of the Signals Technical Committee of the National Committee on Uniform Traffic Control Devices (NCUTCD), and John Hibbard, Transcore, member of Signals Technical of the NCUTCD, for their assistance in reviewing this article for accuracy regarding signal timing issues.
As members of the Environmental Access Committee of the Orientation and Mobility (O and M) Division of AER (Association for Education of the Blind and Visually Impaired), we were pleased to see that a topic as important as pedestrian clearance intervals was addressed in Stevenson (2001). However, we were concerned to see inaccuracies and misleading conclusions in the paper and believe that clarification is necessary.
The techniques of O and M were developed in the 1940s and 1950s, and were appropriate for the state of traffic engineering at that time, when traffic lights changed in a predictable fashion, controlled by mechanical timers. However, with the mass move to the suburbs, the explosion of the number of cars on the road, and the simultaneous advent of computers, the management of traffic began to change and actuated intersections became increasingly common.
Because of these changes, the evaluation of traffic signal timing recommended in Stevenson (2001) will not be adequate at many intersections. At fixed time intersections, Stevenson’s statement that the calculated crossing time reflects a total of the “green and yellow” time is an accurate observation. Stevenson is also correct in his statements about the 4 feet per second (fps) rate used in calculating pedestrian street crossing times, although there are other factors to be considered.
However, Stevenson (2001) does not discuss the nature of the actuated traffic signal and the pedestrian’s active role in determining variable timings at that type of intersection. His recommendation to “use a stopwatch to time the lights” could lead to dangerous errors by blind pedestrians at actuated intersections. This Comment is meant to provide a brief overview of the details of the current situation in traffic signal timing.
Traffic signal timing
There are two main types of controllers used to switch traffic lights: pretimed and traffic actuated.
Pretimed, or fixed time, traffic lights operate with a predictable, regularly repeated sequence of signal indications. Sometimes the traffic signal timing changes based on the time of the day or day of the week, but the sequence and timing of traffic movement is predictable and typically the same at a particular time of day. Often, the major street at the intersection gets more time, or a longer phase, and the minor street gets less time, or a shorter phase. Most signals used to be of this type. They are still very common in downtown areas because traffic volume is predictable, stable, and fairly consistent.
At traffic actuated intersections, the lights respond to the traffic that is present. Detectors recognize the presence of cars in each lane and notify the controller unit of the vehicle’s presence. The length of the light is controlled by a complex communication between detectors and the intersection controller. The green phase programmed for a single car is not likely to be long enough to allow a pedestrian to safely cross the street; it may be as short as six seconds.
At actuated intersections, if the intersection is also designed for pedestrian use, a pushbutton may be installed that the pedestrian must push to get a special longer pedestrian timing, or ped phase. If the button is not pressed, there is no ped phase, so no additional time will be included in the light cycle for the pedestrian.
It is possible for a street or intersection to operate on a pretimed basis at some times, and on fully actuated or semi- actuated control at other times. Also, in case of construction damage to the actuators, or computer failure, the intersection will usually revert to pretimed.
The semi-actuated intersection is the most common type that will be encountered by travelers who are blind. They are very common in suburban areas, usually when a minor street crosses a major one. This type is called semi-actuated because only the minor street has sensors or detectors in it and the major street typically has a green light until a car waiting on the minor street triggers a change in the light.
Typically, a single car will cause the light to change for no more than seven seconds, just long enough for the car to turn onto or cross the major street. Each additional car will add additional time, perhaps two seconds, up to a preset maximum timing that rarely exceeds 30 seconds. The vehicular clearance interval, which is a combination of a yellow change interval and a brief interval of time during which all vehicular signals are red, is provided as the light changes back to green for the major street. There may only be a total “red” time on the major street of as little as six seconds if there is a single car on the minor street. This timing will apply even to major arterial streets, where 6 seconds is not nearly enough time for a pedestrian to cross. Note that nowhere in the scenario just described was there any mention of pedestrians crossing or their timing needs.
On streets where pedestrians are expected, pedestrian phases can typically be actuated by pedestrians, either by pushbuttons or passive detection systems. The “Walk/Don’t Walk” light (pedestrian signal head, or “pedhead”) typically indicates the walk interval for 4 to 7 seconds, followed by a “pedestrian clearance interval,” the amount of time calculated to enable a person walking at a standard rate of speed to reach the other side of the street before the onset of perpendicular traffic. Since the green phase programmed for a single car may not be long enough to allow a pedestrian to safely cross the street, it is essential for pedestrians to request a pedestrian phase by pushing the pedestrian pushbutton.
Some intersections may have a pedestrian button but no pedhead. In that case the pedestrian timing will apply, but there is no visual information provided for the pedestrian. Other intersections have a pedestrian button and a pedhead. There are many traffic actuated intersections in suburban or rural areas that do not have a pushbutton or pedestrian accommodations, and the cycle may not include adequate time for pedestrians to cross.
The messages provided by the pedheads are so widely misunderstood by the general public that they are worth a review here. The “Walk” interval is generally only 4 to 7 seconds long; the signal only says “Walk” or shows the illustration of a walking person for that long. The pedestrian is expected to leave the curb during this interval but is allocated a much longer period—the clearance interval—to complete the street crossing. The flashing “Don’t Walk” or the orange flashing hand symbol is visible during the pedestrian clearance interval. The clearance interval is a calculated period to allow pedestrians to complete their crossing. The solid “Don’t Walk” signal occurs after the flashing “Don’t Walk,” usually in conjunction with the vehicular yellow signal. Pedestrians should have cleared the intersection by that time.
As discussed by Stevenson (2001), to calculate the pedestrian clearance interval, traffic engineers divide the width of the street by 4, which represents the “4 feet per second” (fps) that the average pedestrian walks. If the street is 80 feet wide, the pedestrian clearance interval will be 20 seconds. This is adequate time for a majority of pedestrians to reach the opposite curb, if they leave the curb by the first flash of “Don’t Walk.” The Manual on Uniform Traffic Control Devices, Millennium Edition (MUTCD) does allow and recommend that “where pedestrians who walk slower than normal, or pedestrians who use wheelchairs routinely use the crosswalk, a crossing speed of less than 1.2 m (4 ft) per second should be considered in determining the pedestrian clearance time” (MUTCD, 2001, Section 4E.09 Pedestrian Intervals and Signal Phases).
Implications for O and M specialists and pedestrians who are visually impaired
Complex timing schemes and actuation plans can cause access problems for blind travelers, particularly in the absence of substantial regular traffic volumes. If blind pedestrians try to cross arterial streets using the standard O and M rule of “near side parallel surge” without getting a pedestrian phase, they can be, and often are, caught by short timings. For some travelers, this may be a conscious choice to not bother to try to find a button that is so badly placed as to make it useless. For others, ignorance of the mechanisms of intersection functioning leads to dangerous and uninformed decisions. An understanding of intersection timing and of the proper use of pedestrian buttons is essential to safe street crossings at many intersections today.
The possible variations in intersection timing and actuation make it important to contact the local traffic engineer to get accurate information about signal timing at individual intersections. An O and M specialist may measure the width of a crossing and time the pedestrian clearance interval and thereby determine the pedestrian walking speed assumed in the calculation, as suggested by Stevenson (2001). However, it is not possible to determine, even using a measuring wheel and stopwatch, exactly what the timing of any actuated signal system will be on any specific future cycle.
Stevenson, in encouraging O and M specialists to determine signal timing without mentioning that most signals do not operate in a fixed time mode, encourages them and their students to act under a false assumption that can lead to life-threatening consequences. For example, an O and M specialist may observe an actuated intersection during rush hour, when the signal always lasts 30 seconds, without pushing the pedestrian button, and assume the signal is always 30 seconds whether or not the button is pushed. If the student crosses when the traffic is light but doesn’t push the button, the signal may be only 6 seconds long, and the student will be caught in the middle of the intersection when the traffic begins moving.
Stevenson, in his article, focused on lengthening the pedestrian clearance interval as a strategy to provide safer crossings for blind pedestrians. However, most blind pedestrians do not walk more slowly than the norm. The need for more time to cross is a result of their need to evaluate the status of the light by hearing traffic moving through the intersection and the delay occasioned by having to wait to hear a car travel across the intersection. At intersections with heavy traffic, that delay often means that blind pedestrians are beginning their crossing after they hear the surge of traffic crossing the intersection, which is too late in the pedestrian phase and often well into the clearance interval. At locations with intermittent traffic, pedestrians who are blind may push the pedestrian button and get the phase but have no traffic to cue them that the light has changed. Stevenson’s suggestion to lengthen the pedestrian clearance interval by calculating the pedestrian walking speed at 3 fps does not solve that problem and in many instances will result in unnecessary traffic delays with attendant driver aggression or road rage.
A more appropriate solution is to provide blind pedestrians with information about the status of the traffic light, which will allow them to leave the curb as soon after the onset of the walk interval as it is safe to do so. Actuated systems typically provide visual guidance for pedestrians so they will cross at the phase designated by the engineer for pedestrians. At actuated intersections, information about the onset of the pedestrian phase can be provided to blind pedestrians by Accessible Pedestrian Signals (APS). The technology of APS has changed, and newer systems do not have some of the problems associated with previous types of audible signals available in the United States Modern APS technologies are described in Accessible Pedestrian Signals (Bentzen and Tabor, 1998).
In some cases, lengthening the pedestrian clearance interval is appropriate; in other situations, an APS is needed, and in others there may be a need for pedestrian timing to be added to the signal timing sequence. A combination of all of the modifications just mentioned may be appropriate at some intersections.
As Stevenson (2001) suggests, communication with traffic engineers is a necessary part of teaching O and M skills. This communication requires a basic understanding of the complexity of today’s signal systems and a willingness to work with engineers to evaluate a variety of solutions. If O and M specialists are discussing national traffic engineering standards, as suggested by Stevenson, they should carefully ascertain that they are using the most up-to- date materials. The MUTCD was updated in 1988 and again in 2000, and a revision of the Traffic Engineering Handbook was published in 1999. Referring to an out-of-print 1983 publication, as Stevenson did, in a quickly changing field like traffic engineering, may not provide appropriate information for discussion.
Blindness professionals and individuals who are blind or visually impaired need to learn to speak the language of transportation engineering and transportation planning professionals. To appropriately teach street crossing skills at signalized intersections, O and M specialists must understand the engineering definitions of “cycle,” “phase,” and “interval.” The traffic engineering community is searching for ways to make the streets easier to use for a range of travelers, including pedestrians who are blind or visually impaired, which means that blindness professionals can begin to have a positive impact on how engineers design streets, sidewalks, and intersections.
Bentzen, B.L., Barlow, J.M., and Franck, L. (2000). Addressing barriers to blind pedestrians at signalized intersections. ITE Journal, 70–9, 32–35.
Bentzen, B.L., and Tabor, L. (1998). Accessible pedestrian signals. Washington, DC: U.S. Architectural and Transportation Barriers Compliance Board.
Kuemmel, D. (2000). Accessible pedestrian signals. ITE Journal, 70–3, 40–42.
Pline, J. L. (1999). Traffic engineering handbook, Fifth edition. Washington, DC: Institute of Transportation Engineers.
Stevenson, J. (2001). Pedestrian clearance intervals. Journal of Visual Impairment and Blindness, 95, 237–239.
U.S. Access Board. (2001) Building a true community: Final report, Public Rights-of-Way Access Advisory Committee. Washington, DC: U.S. Architectural and Transportation Barriers Compliance Board.
U.S. Department of Transportation, Federal Highway Administration. (1995). Improving traffic signal operations: A primer. Washington, DC: Institute of Transportation Engineers.
U.S. Department of Transportation, Federal Highway Administration. (2000). Manual on Uniform traffic control devices, Millennium edition. Washington, DC: Institute of Transportation Engineers or [On-line]. Available: http://mutcd.fhwa.dot.gov/kno-millennium.htm.
Janet M. Barlow, M.Ed., COMS, Accessible Design for the Blind, Atlanta, Georgia.
Lukas Franck, M.A., COMS, The Seeing Eye, Washington Valley Road, Morristown, NJ 07960.
Billie Louise Bentzen, Ph.D., COMS, Accessible Design for the Blind. Mailing address: P.O. Box 1212, Berlin, MA 01503; e-mail: Bbentzen@ma.ultranet.com.
Dona Sauerburger, M.A., COMS, 1606 Huntcliff Way, Gambrills, MD 21054; e-mail: Dona@Sauerburger.org.
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