Exceptions to rule that

ďAfter the button has been pushed, the WALK signal starts

the next time that traffic in the nearest parallel lanes goes straight-through.Ē


Excerpted from ďCrossing at Modern Signals

Fall 2005 Newsletter, AER Orientation and Mobility Division


Summary Ė known exceptions to the rule are:

   When crossing a secondary street when both streets are actuated;

   When actuated signals are coordinated with other signals along the street;

   We donít know what other exceptions may exist!






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


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