Predicting
crossing time by observing the timing of the signal
Excerpted from “Crossing at Modern Signals
Fall
2005 Newsletter, AER Orientation and Mobility Division
This
revelation . . . 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.
Some O&M specialists have resorted to
timing the signal phases to predict when it’s 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 doesn’t 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 hadn’t 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 don’t 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.
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.”
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
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