Predicting crossing time by observing the timing of the signal
Excerpted from ďCrossing at Modern Signals
Fall 2005 Newsletter, AER Orientation and Mobility Division
This revelation . . . 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.
†††† Some O&M specialists have resorted to timing the signal phases to predict when itís time to cross.† In general, the strategy works like this.† First, you observe that your walk signal or green light comes on at a certain time after another event, for example 17 seconds after the left-turning traffic on the parallel street starts, or 25 seconds after the perpendicular traffic starts.† Then, when the observed event occurs, you start counting seconds until you assume your signal starts, and you begin crossing.
††† In addition to requiring the ability to accurately measure the passage of time, this strategy requires that the timing of the cycle is reliably consistent and predictable.† There are two problems with this strategy:
1) No traffic signal cycle is reliably consistent and predictable.
†††† Of course the timing at actuated signals will vary with each cycle, depending on how many vehicles are in the various lanes.† But even fixed-time signals can be programmed to vary at different times of the day or week to handle different traffic loads.
†††† And even if you had assurance from the traffic engineers that the signal is fixed-time and doesnít vary during the day or week, it is still not reliable because the timing can easily be changed without your being aware of it.
2) It is practically impossible to anticipate every conceivable scenario that could undermine the strategy
†††† One example of a strategy for crossing which presumably considered every possible scenario but which overlooked an important factor was developed for crossing an arterial which is 7 lanes wide.† The problem was insufficient traffic from the parallel side street to use as a cue to cross.† Although the signal is semiactuated, the strategy carefully took this into consideration.
†††† It was observed that the arterial being crossed had a minimum time for its green signal, so that when the pedestrian button was pushed to cross it, the signal did not change until the arterial had its minimum time of 45 seconds.† That is, if the pedestrian button is pushed less than 45 seconds after the signal for the arterial is red, the signal will not change to red again until the arterial road has had at least 45 seconds of green.† So, when a minute passes without the signal for the arterial road being red, its signal will turn yellow as soon as the pedestrian button is pushed and, exactly 10 seconds later, the walk signal to cross it begins.†
†††† Using this strategy, then, the pedestrian would wait until the arterial road had the green signal for at least a minute, then push the pedestrian button and count ten seconds while preparing to cross.† At the end of ten seconds, the pedestrian simply assumes that the walk signal is on, and starts to cross.
†††† It would seem that everything that could go wrong, such as the effects of actuation, was considered and addressed.† Unfortunately, something that hadnít been anticipated could make it go very wrong.
†††† This strategy requires that pedestrians realize whenever the signal for the arterial changes to red, because they must wait at least a minute after the signal is red before they push the pedestrian button.
†††† But the signal can change to red without the blind pedestrian realizing it, as explained here:† After the arterial road has had its minimum of 45 seconds of green signal, whenever a car passes over the sensors on the side street, the signal for the arterial road turns to yellow and then red.† This happens even when the car is not coming from the side street, but rather is turning left from the arterial into the side street and cuts the corner so closely that it rides over the sensors.† This happened several times during the time I was there with my client (about 6 hours altogether).
†††† Rather frequently, then, even when there is no car waiting on the side street, the signal for the arterial changes to red.† This would be no problem if the traffic on the arterial road stopped so the blind pedestrian would realize the signal had changed.† She would then know that she has to wait another minute before pressing the pedestrian button.† However, if there is a gap in traffic on the arterial during the red signal, no cars will stop, and the blind pedestrian will not realize its signal had changed to red and back to green again.
†††† This could happen very easily.† If the signal for the arterial turns red during a gap in traffic that is 10 seconds or longer, the blind person will never know the signal had changed.† Because of traffic signals within a half mile in each direction along this arterial, there are many gaps in traffic on the arterial that are much longer than 10 seconds, even during busy times of the day.† During one 15-minute observation of traffic on a weekday afternoon, there were 26 gaps -- almost two every minute.† The gaps each lasted from 5 to 22 seconds, with the average gap being 13 seconds.† Seventeen of the gaps were 10 seconds or longer, which is more than one such gap every minute.
†††† If a car from the arterial drove over the sensors at the beginning of one of these gaps (which is very likely if the driver was waiting for a gap to turn left into the side street), the signal would turn red for the arterial road without the pedestrian knowing it because there would be no cars on the arterial that stop.† If the pedestrian pushed the button within a minute of that car driving over the sensors, the walk signal would NOT come on in ten seconds, it would come after the arterial road had 45 seconds of green.
†††† So, because the pedestrian would be under faulty assumptions and using a strategy based on unpredictable features, she might start to walk across a seven-lane street when her signal is red and the traffic on the street she is crossing has a green signal.
†††† This is just one scenario in which assumptions about this signal could be erroneous.† There are a number of ways by which actuated signals become dysfunctional and donít follow normal patterns.† For example whenever the actuation wires in the street are cut during road repair or construction, the signal reverts back to fixed-time until it is repaired again, which often takes months.† This 10-second strategy would not work then because the walk signal would not come 10 seconds after the button is pushed, it would come at regular intervals regardless of the button.
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
†††† 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.
J. (2005, Winter).†
Crossing strategies for modern signalized
of the Orientation and Mobility Division†
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