Exceptions to rule that
“After the button has been pushed, the WALK signal
starts
the next time that traffic in the nearest parallel lanes goes
straight-through.”
Excerpted from “Crossing
at Modern Signals
Fall
2005 Newsletter, AER Orientation and Mobility Division
Summary – 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 don’t
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.
I’ll 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 isn’t 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 didn’t 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, wasn’t reliable -- convinces
me that it’s 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 hadn’t necessarily considered.
As Barlow, Bentzen and Bond wrote (2005, p.
597):
“The 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.”
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