Predicting crossing time by using stopped perpendicular traffic after observing the traffic pattern / signal cycle


excerpted from Crossing at Modern Signals, by Dona Sauerburger, COMS

Fall 2005 Newsletter, AER Orientation and Mobility Division


†††† What follows is an example of a situation which illustrate how the strategies of timing, predicting traffic patterns, and using stopped perpendicular traffic as a cue can backfire and lead people to start the crossing when they are at risk because they are crossing against the light.It was at an intersection where my client wanted to cross a 7-lane street (Piney Orchard Parkway) with a 4-lane street to his right (Waugh Chapel Road).Piney Orchard is a north-south street and he wanted to cross it from the SE corner to the SW corner.Although Piney Orchard is the main street, Waugh Chapel is also a major route for thousands of residents coming home from shopping or work.There is normally traffic on it at all times of the day.

†††† We observed the traffic over an extended period of time, and noted that the pattern was the following:

1st: left-turning traffic from Piney Orchard (there was always southbound traffic waiting to turn left to go east);

2nd: all traffic in both directions on Piney Orchard;

3rd: left-turning traffic on Waugh Chapel (if there is any)

4th: all traffic in both directions on Waugh Chapel.

†††††††† Because the first traffic movement from his parallel street Waugh Chapel might be the traffic beside him with a green arrow to turn left across his path, my client used the effective strategy of starting to cross with the traffic in the nearest parallel lanes -- that is, the Waugh Chapel traffic coming east from across Piney Orchard.

†††† He expected that after all the traffic on Piney Orchard stopped, the next phase of the cycle would be Waugh Chapel, and he would start to cross when he heard the Waugh Chapel traffic coming from 7 lanes away, on the other side of Piney Orchard.Sometimes this was the first parallel movement.

†††† However, whenever Piney Orchard traffic had been moving for its minimum timing (about 45 seconds) and no vehicles had approached from Waugh Chapel, if a southbound vehicle on Piney Orchard approached from the north to turn left into Waugh Chapel, the northbound traffic on Piney Orchard would be stopped for just a few seconds and allow the left-turner to proceed onto Waugh Chapel, then they would resume again.

†††† When left-turning traffic from very wide perpendicular streets pass blind people waiting on the corner, it often sounds like parallel traffic coming from the other side of that wide, perpendicular street.So when the scenario that I just explained occurred, all three lanes of traffic on the perpendicular street Piney Orchard stopped in front of my client, and then he heard what sounded like traffic coming from his parallel street Waugh Chapel on the other side of Piney Orchard.

†††† But that wasnít Waugh Chapel traffic -- it was left-turning traffic from his perpendicular street, Piney Orchard!He couldnít hear that the traffic on Piney Orchard going south was NOT stopped, and he didnít know that the traffic in front of him was about to get the green signal again, and so he almost started crossing seven lanes against the light! We worked hard to have him distinguish the sounds of left-turning perpendicular traffic from that of parallel traffic, and eventually he was able to do it.

†††† The problem with trying to predict traffic movement based on our understanding of the system is that we donít have engineering degrees and even if we did, we donít know how these engineers designed this signal.Traffic engineers have explained to us that there is no way that even they could predict what will happen at any given intersection based only observation, no matter how long the observation is.



We cannot predict or consider all the potential factors!

†††† I think very few of us -- even traffic engineers -- understand the workings of the signals enough to predict everything that could happen.I have studied, observed, and dealt with actuation and the modern traffic patterns for about 10 years, and attended sessions with traffic engineers to try to understand how the system works.Yet every time I think I finally understand actuation and how it works, I am dismayed to find an exception to the rule, a situation which I had not predicted.This happened just two weeks ago while I was presenting on this subject to the Northeastern chapter of AER, when I still complacently thought I could outwit these signals and develop reliable strategies to deal with them.

†††† Iíll start at the beginning.I had always taught that if you push a pedestrian button when your signal is green, the walk signal will not come on right away, it will come on at the beginning of the next cycle.One day as I was teaching this to a client, we pushed the button while our signal was green and oops!The walk signal came on immediately!

†††† I called the engineer and learned that this can happen when you cross secondary streets at fully actuated signals (that is, where there are walk signals and pedestrian buttons for both streets).In these situations, if the main street has the green light (for crossing the secondary street) and no vehicle or pedestrian is waiting to cross the main street, the walk signal does comes on as soon as you push the pedestrian button to cross the secondary street.This fact isnít well known, even among traffic engineers -- in 1999, at a meeting of the Metropolitan Washington O&M Association with traffic engineers from state and county jurisdictions, most of the engineers didnít realize this was true. The problem was solved at this intersection when APS were installed a few weeks after we requested them.

†††† Meanwhile, I developed a strategy which I thought would deal with situations where we cannot get an APS.The strategy is to first push the button to cross the main street, then push the other button to cross the secondary street.Because you put in a request to cross the main street first, it will not give you the walk signal to cross the secondary street until it has responded to the request to stop traffic on the main street and allow a pedestrian to cross it.So you can be assured that you have the walk signal to cross the secondary street when the main street traffic begins again.

†††† However an O&M specialist at the Northeast AER conference told me that he noticed that this strategy isnít reliable either.Main streets are usually given a minimum of time before their traffic will be stopped to respond to a request to cross them, and if the main street has enough time left to allow a pedestrian to cross before its minimum time is up when you push the button, it WILL give you the walk signal to cross the other street immediately, even after youíve pushed the button to cross the main street!

†††† This revelation -- that a strategy, which I had assumed took everything into consideration, wasnít reliable -- convinces me that itís impossible to comprehend and take into consideration all the possible mechanisms which could affect the traffic patterns and timing of signals.For example, one mechanism which makes an exception even to our traditional rules (and which curls our toes to think about!) is a system that exists where actuated signals are coordinated with other signals along the road.In that situation, if you push the pedestrian button to cross the main road, the walk signal may not come on the next time the parallel traffic gets the green signal!This happens whenever there is time to allow a few cars to enter the intersection from the secondary street without messing up the system, but not enough to allow a pedestrian to cross.So the vehicles get a green light for a few seconds but the pedestrians do NOT get a walk signal, nor enough time to cross the street!

†††† How can we teach our consumers to use timing or patterns to predict when their walk signal will begin when the system is so complex, and we continue to discover exceptions to the rules?The possibility for error is too great because of exceptions of which we were not aware, and which even the traffic engineers hadnít necessarily considered.As Barlow, Bentzen and Bond wrote (2005, p. 597):

†††† ďThe lack of awareness of laws and signal-timing issues puts blind pedestrians at risk of injury and O&M instructors at risk of being considered liable for giving clients incorrect information.Updated techniques for evaluating intersections, using pedestrian pushbuttons, aligning to cross, and determining the appropriate crossing time are needed.However, at many intersections, strategies and techniques will not resolve the difficulties or provide enough information for crossing safely without access to the signal information.Ē


Barlow, J., Bentzen, B., and Bond, T.(2005). Crossing Strategy for Modern Signalized Intersections.Journal of Visual Impairment and Blindness, 99, pp. 587-598.


Frieswyk, J. (2005, Winter).Crossing strategies for modern signalized intersections.Newsletter of the Orientation and Mobility Division[Alexandria, VA: Association for Education and Rehabilitation of the Blind and Visually Impaired], 11 (2), pp. 16-17


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