The in basket: Dr. Craig Benson writes, “Perhaps you can
help me crack the code of the traffic lights along Highway 305 in
Poulsbo. I read in the Sun that as part of the revamping of the 305
corridor the lights were supposed to be timed to provide for
smoother traffic flow and less stopping, to save gas.
“Since I live off Hostmark and often shop at College
Marketplace,” he said, “I have occasion to pass through all six
lights in between at various times of the day and night, sometimes
with no other traffic interfering, but rarely at rush hours. I’m
pretty sure I’ve never made it through all the lights without
stopping.
“I’ve tried setting my cruise control right at the posted limit,
a little higher, a little lower, accelerating faster and slower
from the lights, everything I can think of to pass through the
lights as timed, without success.
“This is an every day occurrence for me,” he said, “and probably
for hundreds if not thousands of others – those stops add up! Can
you find out from the powers that be what I’m supposed to be doing
to get through the corridor without making unnecessary and
inefficient stops?”
The out basket: The state gets this inquiry enough that it has
posted a discussion of the limitations of synchronization on its
Web site, at
wsdot.wa.gov/Operations/Traffic/Signals/signal_coordination.htm.
After noting that synchronization requires the same number of
seconds for each signal to serve all of its various traffic flows,,
it poses a question of itself: “Does this mean I will never have to
stop for a red light?”
“Unfortunately, the answer to this question is No,” it replies.
“There are many reasons why, even when traffic signals are
coordinated, you will still have to stop at red lights.”
Among the reasons:
– Pedestrians. Whether those on foot consume some of the
time devoted to the various movements affects the amount of
time for other movements.
– Side streets. Even where through traffic is given preference,
such as on Highway 305, making for long waits for a green, the goal
is to make sure all cars waiting when their queue gets a green
light are served. The number of cars crossing affects the main line
timing.
– Left turns. The amount of time devoted to them and the number
of vehicles waiting to turn subtracts seconds from the main
line.
– Two-way traffic flow: Coordinating flows in opposite
directions is difficult, especially if the signals in the corridor
are spaced differently. “If the spacing is not equal between
traffic signals, the green lights may only ‘line up’ well in one
direction,” the site says. “When this happens, the green lights
will normally ‘line up’ better in the direction with the most
traffic. The traffic in the other direction may have to stop.”
– When you drive. Many coordinated system are taken out of
synchronization at night and on weekends.
I’ve had to abridge the information on the Web site, for space
reasons. Look it up for a more detailed explanation.
