"Modern" Signalized Intersections

Janet M. Barlow, COMS
Certified Orientation and Mobility Specialist
Accessible Design for the Blind



Lukas Franck, COMS, GDMI
Senior Special Projects Consultant
The Seeing Eye

Traffic signals have gotten smarter and smarter. As a consequence, pedestrians who are blind and visually impaired need to reconsider the strategies they have traditionally used for crossing streets. Those strategies may need an update. First, it’s important to understand the way traffic signals work.

Traffic signals change in two ways, either mechanically or by computer. Computers control most traffic signals now. The computers that control intersection timings are usually called “controller assemblies” (CA). Sometimes traffic signals are controlled from traffic management centers located miles from the intersection.

Some intersections have extremely complex programming that allows the intersection timing to change constantly based upon demand. There are two basic types of programs used at traffic signals. An individual signal may be managed by both programs at different times of day. This depends on the decisions made by traffic engineers who strive to make the intersection function efficiently.

Pretimed (fixed time) traffic signals change with a predictable, regularly repeated sequence of signal indications. For example, a light that changes every 30 seconds is a pretimed or fixed timed light. The vehicles on the major street get more time, in rush hour, the time allocated to the major street can be even longer. Generally, pretimed signals are very predictable. Most signals used to be of this type. They are still common in downtown areas because traffic volume is predictable, stable, and fairly consistent. In some cities, the old style mechanical CA are still in use and the gear mechanisms can be heard as they click into place.

Traffic actuated signals always have CA (computers) that respond, either to the traffic that is present, or to commands from a remote Traffic Management Center. For example, if there is only one car in the northbound lanes, the northbound signal may be green only for enough time to allow that car to cross the intersection and if there’s no pedestrian (or if the computer that controls the intersection doesn’t KNOW there is a pedestrian there), the signal may not provide enough time for a pedestrian to cross the road. At rush hour you may have to wait longer than usual for the longer pedestrian signal because the “Traffic Command Center” decides to delay your long pedestrian phase to keep traffic moving more efficiently.

Detectors

How do intersections “know” that there are cars or pedestrians needing a turn? At actuated intersections, detectors are installed in all or some of the lanes that recognize when there are vehicles present. Vehicle detection is accomplished by the use of metal cables laid in the street, which sense when a vehicle drives over them. This method of detection is called an induction loop.

It is also becoming increasingly common to use computerized video cameras, mounted overhead, to detect vehicles, and less commonly, pedestrians.

The detectors sense the presence of a car or cars. They send that information to the CA that makes a decision, based upon its programming, about how long to allocate green to each street. A single car waiting on a side street to cross or turn onto a major six lane street may get less than seven seconds of green light, followed by a few seconds of yellow, and then all red, to accommodate that movement! If there were five cars waiting on that side street, more time would be allotted, perhaps 15 seconds of green, plus the several seconds of yellow and all red time. If there were no cars on the side street, the CA would know that too, and the light would never change!

Turn Arrows

At many major intersections, safety may dictate that cars making a left turn have a dedicated point in the signal cycle just for them. The drivers know that it is their turn because they get a green “turn arrow” instead of a regular green light. When turning on an arrow, those vehicles have the right of way, even when crossing the crosswalk. The WALK SIGN will not come on until after the turning phase is over for that reason. Drivers will not be expecting pedestrians to be crossing during the arrow. However, this can be confusing for pedestrians who are blind or visually impaired listening to the cycle because, if there are no cars that need to turn, that ‘phase’ may be skipped over. The CA, getting no information from the sensor, skips the phase. A pedestrian who has a visual impairment may therefore encounter different timings at an intersection at different times. It is easy to become confused, because the initial movement of cars on the parallel street may in fact be one of these dedicated turning movements and not the through parallel traffic movement that provides a cue to the beginning of the cross walk signal. Don’t get caught!

Pedestrian Timing and Pedestrian Signals

If the intersection was designed for pedestrian use, there may be a visual pedestrian Walk/Don't Walk signal, sometimes called a “pedhead”, short for pedestrian signal head. To get a walk signal, you must push the pedestrian button to get the “pedestrian timing”, which is usually longer than the time allotted for vehicles. Especially when the main street is very wide, the green phase programmed for a single car will not be long enough to allow a pedestrian to safely cross the street. In that situation, it is essential for the pedestrian to push the pedestrian pushbutton in order to have enough time to walk across the street. Rather than five to seven seconds for a single car, a pedestrian may get twenty seconds or more to cross the same street, but only IF the CA "knows" the pedestrian is there.

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 pedestrian is expected to leave the curb during this interval, but is allocated a much longer period to make it across the street.

When a flashing DON'T WALK or ORANGE HAND is visible, it is called the "pedestrian clearance interval". It lasts much longer than the WALK interval. It is often misunderstood, but the intent of flashing DON'T WALK is to discourage pedestrians from entering the intersection and starting to cross when there is not sufficient time to complete the crossing. The pedestrian who is in the middle of the street generally has time to complete the crossing after the flashing DON'T WALK begins.

The solid DON’T WALK signal occurs after the flashing DON’T WALK, usually at the same time as the vehicles’ yellow signal. Pedestrians should have cleared the intersection at that point. There is also usually a "vehicular change interval" when the light is red in all directions.

In many areas, a visual countdown is now being incorporated into the pedhead. This signal is usually orange numbers counting down the number of seconds left in the flashing don’t walk signal, displayed beside the orange hand. In some locations, the countdown begins during Walk, so the orange numbers may be changing beside the white “walking man” WALK symbol. This is technically incorrect (not allowed by the Manual on Uniform Traffic Control Devices (MUTCD)). Where this is done, it has been confusing to some pedestrians with low vision who have mistaken the countdown for the flashing don’t walk (at the same time the WALK is displayed).

In some cases there may be a pedestrian button but no pedhead. In that case, pushing the button will change the signal timing to allow enough time for pedestrians to cross but the pedestrians are expected to rely the vehicle signals as indications about when it is safe to initiate a crossing. There is no separate information provided for pedestrians.

Here are two other variations that will require an Accessible Pedestrian Signal (APS) if a pedestrian who is blind is to have equal benefit from pedestrian-friendly designs!

Leading Pedestrian Intervals

Sometimes pedestrians can be given a head start, called a leading pedestrian interval (LPI). The pedestrian WALK may be displayed 3 to 5 seconds before the vehicles get a green signal. This is to allow pedestrians to begin crossing before cars begin turning right on green. However, without an APS, pedestrians who are blind may not know about an LPI, and might begin crossing with near side parallel traffic – precisely when drivers are NOT expecting them to go.

All Stop (Scramble or Barnes Dance) Light Signals

At other signals, the pedestrian timing is provided at a time when all vehicles have a red signal, technically called an “exclusive pedestrian phase”. Again, without an APS, pedestrians who are blind may not know when the pedestrian phase begins, or may begin crossing when a vehicle stops for another reason. Or the pedestrian who is blind may cross with the parallel traffic, essentially illegally since pedestrians are required to “obey pedestrian signals when present”, in most states.

Strategies - Crossing with “near side parallel traffic”

The traditional technique used by pedestrians who are blind is to initiate the crossing when there is a surge of parallel traffic movement. Travelers may delay beginning the crossing until they are sure that the traffic is going straight and not a vehicle turning right on red (Allen, et al, 1997; Hill and Ponder, 1976; LaGrow & Weesies, 1994; Jacobson, 1993; Willoughby & Monthei, 1998). With current signalization patterns, the above strategy needs refinement and additional detail, to focus on only certain lanes of traffic.

The pedestrian walk signal is usually coordinated with the movement of the traffic in the parallel through lane nearest to the pedestrian, also called the “near side parallel” traffic. If you received orientation and mobility (O&M) training a number of years ago, you may not have heard or used that strategy. It’s intended to prevent beginning crossing while cars are turning across the crosswalk on a left turn arrow.

Traffic in the “near lane” may be traveling in the same direction as the pedestrian or may be coming toward the pedestrian from across the intersection. Mainly, when crossing with the parallel street on your right side (clockwise crossing), the nearest lane is the traffic coming toward you from across the intersection, rather than the traffic moving in the same direction as you are going. When crossing with the parallel street on your left side (counterclockwise crossing), the near side parallel traffic is going the same direction, coming from behind you.

Vehicles may still be permitted to turn left after the near side traffic is moving, but they no longer have the right of way and are required to yield to oncoming traffic and to pedestrians in the crosswalk. When traffic in the near parallel lane is moving and steady, it also can block left turning traffic from crossing the crosswalk.

Although this technique can prevent crossing into traffic turning with a left turn arrow, and timing usually coincides with the WALK, there are intersections where there is intermittent or no parallel traffic and the signal status cannot be determined well enough by traffic sounds. At such intersections, including those with exclusive pedestrian phasing or an LPI, an APS is needed to provide access to walk signal information.

Variations on a theme – advocacy

As was previously mentioned, it is also possible for a street or intersection to operate on a pre-timed basis at some times, and on actuated control at other times. Also, in case of construction damage to the detectors, or computer failure, the intersection will usually revert to pre-timed mode. The possible variations make it important to locate and use pedestrian pushbuttons, to carefully analyze the traffic, and to cross with the ‘near side parallel traffic’ movement. However, even doing all those things right may not result in crossing at the ‘right time’ at every intersection. It may be helpful to call your local traffic engineer to get the details when using an unfamiliar intersection. It may help to learn the theory and "jargon" so that you can solicit the information from traffic engineers. Please see the glossary for some key terms.

The fact that signals can be programmed to switch back and forth between fixed timed and actuated modes has already been discussed, as has the fact that sometimes traffic signal timings can be changed in real time from remote Traffic Management Centers.

Right Turn on Red continues to complicate the street crossing task. Particularly where there are heavy right turn volumes, pedestrians who are blind will often find it difficult to know precisely when the pedestrian signal changes in their favor, while the pedestrian who is sighted, having access to that information visually, will have no problem.

In a nutshell, the computerization of intersection control means that traffic engineers have flexible tools at their disposal to make traffic move more efficiently. This same flexibility can make the street crossing task more complicated for pedestrians who are blind. This both explains and supports the current concern and advocacy for APS. Access to visual street crossing information is imperative when there is so much flexibility and variety in traffic control.

Summary

For all of the reasons discussed in this chapter, it is important to understand modern intersection design. In many instances this information may lead the reader to advocate for an APS at a particular location.

Technology has had an impact on APS as well, and more information on this important topic can be found in the Chapter on Accessible Pedestrian Signals.

Bibliography

Allen, Gary. “From Knowledge to words to wayfinding: Issues in the production and comprehension of route directions.” S.C. Hirtle and A.U. Franks, eds. Spatial Information Theory: A Theoretical Basis for GIS. International Conference COSIT '97, Laurel Highlands, Pennsylvania, USA, October 15-18, 1997. Proceedings (Lecture Notes in Computer Science).

Hill, Everett W. and Purvis Ponder. Orientation and Mobility Techniques: A Guide for the Practioner. New York: American Federation of the Blind Press, 1976.

LaGrow, Steven and Marvin Weesies. Orientation and Mobility Techniques for Independence. New Zealand: Dunmore Press, nd.

Jacobson, William H. The Art and Science of teaching orientation and mobility to persons with visual impairments. New York: American Federation of the Blind Press, 1993.

Willoughby, Doris M. and Sharon L. Monthei. Modular instruction for independent travel for students who are blind or visually impaired-preschool through high school. Baltimore: National Federation of the Blind, 1998.