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 (
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 wasn’t Waugh Chapel traffic -- it
was left-turning traffic from his perpendicular street, Piney Orchard! He couldn’t hear that the traffic on Piney
Orchard going south was NOT stopped, and he didn’t 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 don’t have engineering degrees and even if we did, we don’t 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.
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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