Mats,
Please let this serve as my
formal response to issues that you have raised regarding the duration of the
yellow light phase for the signal located at the intersection of Lombard Ave.
and Allen Blvd. As you are aware, you provided video of this signal that
captured a total of 9 cycles of the yellow light which varied in duration from
3.48 seconds to 3.36 seconds and averaged 3.41 seconds. Since we last
talked, I have secured and reviewed two hours of video at the same location from
Redflex. This video represents over 100 cycles of the same light with the
duration of the visible yellow light varying from 3.36 second to 3.52 seconds
and averaging 3.44 seconds (summary attached). Using these values it
appears that the actual visible yellow phase averages 0.06 seconds shorter than
the programmed value and at most is 0.14 seconds shorter. In all cases
however, the yellow light phase is longer than the minimum (3.2 seconds) as
described below.
The city programs the yellow
phase time and the all red time in its controllers based on the values
recommended by ODOT unless, in the opinion of the city’s traffic engineer, a
longer period is warranted. In the case of the east and west bound legs on
Allen Blvd. these values are 3.5 seconds for the yellow phase and 0.5 seconds
for the all red phase since Allen Blvd. is posted for 3o mph and there have not
been any safety issues that warrant larger values. This value for the
yellow phase is greater than the 3.2 seconds derived from the formula
recommended for use by the ITE (Institute of Transportation Engineers) and is
considered to be a minimum by the city.
In response to your inquiries I
have talked with technical staff at the city and at ODOT in an attempt to better
understand the operation of signal equipment in Oregon. I have learned
that the acceptable tolerance of actual phase time for the combination of new
controllers and load switches is +/- 0.1 second compared to the programmed
value, a tolerance that meets ITE specifications. This tolerance is not
the result of inaccuracies in the controller’s internal clock as I originally
reported, but is instead an acceptable overall tolerance in the combined
controller, software and load switches when bench tested while
new.
This tolerance does not take
into account the acceptable tolerances for new red, yellow and green lamps that
are mounted in the signal heads. Per ITE, the lights must reach 90% of
full illumination within 0.075 seconds of the application of power from the
controller and must fade to black just as fast. In the worst case
scenario, if the green light went black instantaneously and the yellow light
took the full 0.075 seconds to illuminate, and the controller only provided 3.4
seconds of yellow phase, the resultant visible yellow phase could be as short as
3.25 seconds. Obviously this is an unlikely event, but it does demonstrate
the limitations of the equipment that is available and should provide insight as
to why the city does not program its controllers to the minimum value
recommended by the ITE. One item that I want to stress is that all the
preceding discussion applies to all intersections in the city independent of the
existence red light cameras.
While it is true that the
duration of the yellow phase measured by yourself and Redflex is within
equipment tolerances, greater than the minimum values recommended by the ITE,
and most importantly, adequate in the opinion of the city’s traffic engineer who
is empowered by the ITE to use professional judgment, I do believe that there is
room for improvement. As you have previously noted, there is often a delay
between the green light going dark and the yellow light becoming visible.
This may be indicative of the capacitor in the yellow lamp drying out so that
there is less charge available to illuminate the yellow lamp quickly.
Based on my observations of the video, replacing the yellow lamps could result
in an increase of between 0.03 and 0.06 seconds in the duration of the
visible yellow light so this is something we plan to do in the near
future.
Another issue that has come to
light as a result of your observations is the information that the city provides
about signal timing on the Photo Red Light Enforcement FAQ’s web page. As
you have pointed out, the information on the web page reports the programmed
values for the yellow light duration which could be interpreted as guaranteed
minimums. I will be recommending revised language on this page that will
more clearly inform the public about the variance between programmed time and
actual time.
Finally, you have requested that
the city consider following what appears to be Virginia state law by providing a
0.5 second grace period and not ticketing individuals that have entered the
intersection less than half a second after the light turns red. If the
city were to make this adjustment, cars could lawfully enter the intersection
the moment that the opposing lights turned green since the all red phase is 0.5
seconds throughout the region. In order to adjust for this the city would
have to increase the all red phase at these four signals to give these cars time
to clear the intersection which would make these signals operate fundamentally
different than all others in the region. This is contrary to the city’s
goal to operate these signals exactly like the others in the city and throughout
the region. Adding this grace period would also put the city in the
position of ignoring evidence of unsafe behavior that could jeopardize the
safety of the public. This is not consistent with the underlying goal of
the red light camera program which is to improve intersection safety. For
these reasons I am not willing to make this recommendation to the mayor or
council.
I want to thank you for bringing
these concerns to our attention. Ultimately you have given the city a way
to more easily monitor the performance of the signals that are located at the
red light camera intersections as well as given us valuable feedback on the
information that we are providing the public. Please feel free to contact me should you have any
questions.
Sincerely,
Peter
Arellano
Public Works
Director