HomeMy WebLinkAbout07/06/2010 03C-1 Citizen Service Request Responsel RE: Traffic Signal Timing City Council Information •
Memorandum
June 30; 2010
To: The Honorable Mayor and Members of the City Council
Dick Zais, City Manager
From: Joe Rosenlund, P.E.
Streets & Traffic Operations Manager
Re: Response to Council Request Regarding Traffic Signal Timing from Council
Meeting of 5/4/2010
During the recent council discussions regarding red Tight camera enforcement, many of
those commenting on the plan stated that they believed the yellow clearance intervals at
intersections were too short. The following information was presented to the Transit and
Transportation Planning Committee on June 17, 2010 and received their full approval of
the methodology explained herein.
411 The City of Yakima Traffic Operations Division follows recommended guidelines for
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determining the length of yellow clearance interval. Longer change intervals detract
from the available intersection green time and are only needed if there is a greater
potential for vehicle or vehicle- pedestrian conflicts between signal phases. This is the
classic case of balancing traffic capacity versus traffic safety.
Yellow times vary between 2.5 seconds and 5 seconds; most situations call for a yellow
time between 3 seconds and 4 seconds depending on the speed and the size of the
intersection. Too long yellow times encourage drivers to utilize the yellow time as part
of the green cycle where too short does not give drivers time to react safely to the
change in .signal indications. It is desirable to set a minimum yellow interval based on
engineering judgment and then adjust the change interval using an all -red interval if
needed.
The all -red intervals should not be less than 0.5 seconds and woutd normally be limited
to 2.0 seconds. The determination of the all -red interval should be based on the factors
noted above, calculated values, .intersection observations, vehicle clearance practices
at comparable intersections, and engineering judgment.
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The recommended formula for determining an appropriate change interval is:
Y +R= t +V /(2a ±2Gg) + (VV +L)N
Where:
Y = length of the yellow interval
R = length of the all -red interval
t = driver perception /reaction time, recommended at 1.0 seconds.
V = velocity of approaching vehicle in feet/second, recommended that the 85 percentile
signal approach speed or the posted speed limit, converted to feet/second, be used.
a = vehicle deceleration rate, recommended as 10 feet per second2.
g = acceleration due to gravity at 32 feet per second2.
G = grade of the signal approach in percent divided by 100 or 2 percent is 0.02. A
downhill grade results in a negative term, i.e., -0.02.
W = width of intersection measured in feet from the near side stop line to the far edge of
the conflicting traffic lane along the vehicle path.
L = length of vehicle on clearance, recommended as 20 feet for passenger cars.
For example the yellow + all -red interval on Nob Hill Boulevard and S. 1 Street should
be :
Y + R = 1.0 + 51.33/((2 *10) + (2* (32 *0) + (83 + 20)/51.33
Y + R = 1.0 + 2.57 + 2.0
Y + R = 5.57 seconds, rounded to 5.6 seconds
Since most motorists drive at a range of speeds and react to things in different ways,
there is not one "correct" or "safe" duration for the yellow /all -red interval light.
One factor of red light running that Traffic Operations is attempting to address is signal
visibility. This comes in two forms: size of the signal heads and location of the signal
heads. The City of Yakima has numerous locations where the signal indicators are 8-
inch instead of 12 -inch. Size of the signal heads in providing .good visibility particularly in
bright sunlight, fog and for driver's with poorer vision. The latest MUTCD revision calls
for replacement of most 8 -inch signal heads within next 7 years. We are converting to
the larger heads as part of regular equipment replacement and when traffic signals are
installed or reconstructed.
When we construct or reconstruct traffic signals, we have been adding a third signal
head on the pole for each traffic movement. The third head is normally mounted on a
signal pole at the edge of the road. The third head provides two benefits: it allows
drivers to the signal indication sooner if they are traveling behind a larger vehicle and it
gives drivers an easier, more visible indication to watch when the sun is positioned
behind the overhead signals.
These are the best engineering fixes we can provide to reduce red light running.
Enforcement and education are the other two sides of the traffic safety pyramid. We can
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help with enforcement by installing blue signal indicators that allow police to enforce red
Tight running more safely and .efficiently. The blue signal indicators are wired directly to
the red signal indicators and mounted on the backside of the signal heads. The
indicators allow police officer to sit on the downstream side of the traffic signal, still be
able to observe the red signal indication and the vehicle to determine if the vehicle ran
the red Tight, and then pursue the vehicle without having to cross through conflicting
intersection traffic. •
The third part of the traffic safety triangle is education. Education can be provided
through Public Service Announcements (PSA) and by providing speakers to schools,
drivers' education classes or civic groups. We are available to provide speakers for any
issue related to traffic upon request. Some research will be required to determine the
availability of appropriate PSAs.
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