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Contributed by: Brad Bettag on 5/22/2008

So if you are not aware, both the Post and the News ran a story on how 4 intersections in Denver were going to be getting cameras installed at them to help decrease the amount of cars that run the red lights at these intersections.

The intersections are:

36th and Quebec
6th and Lincoln
8th and Speer
6th and Kalamath

So aside from the fact that drivers should be cautious at these intersections, there has been some recent debate on the forum boards on the Denver Post article here:

http://www.denverpost.com/news/ci_9339606

As a future prospective traffic engineer, I would like to take this time to clear up a few misconceptions and inform people.

Firstly the argument that increasing the yellow time to cut down on red light running is bogus.

The formula for Yellow Timing as provided by the MUTCD and used by almost all transportation departments is:

Tmin = delta + (W + L) / Uo + Uo / (2 * a)

Where:
delta is the average human reaction time (usually 1 second)
W is the intersection width
L is the average length of vehicle
Uo is the speed limit on approach
a is the braking (deceleration constant)

a is usually assumed to be = to 11.2

Using this formula at a typical 35 mph intersection with a width of 50 feet and an average vehicle length of 20 feet you get:

Tmin = 1.0 + [(50 + 20) / (35 * 1.47)] + [(35 * 1.47) / (2 * 11.2)]
Tmin = 4.657 or about 4 3/4 seconds

Yellow times usually range from 3 seconds to 5 seconds with a uniform 2 second all red time.

However, this is only part of the problem, with an increased yellow time, dilemma zones increase. These are areas where cars enter at a particular time and they cannot make it through the light in time and are also going to fast so that they cannot stop in time. This is due to the fact that yellow times are not uniform. When you see a yellow do you know that it is going to be 4 seconds? What about 2 seconds? The longer the yellow time the more you will get people who think that they can make it. This may actually increase the number of people who run the red light because they think that they can make it before the light turns red. So changing the yellow time from the standard 3 seconds may not be the way to go either. At least with a standard 3 second time drivers will know exactly how long the yellow will be and will be familiar enough to know whether they can or cannot make it. If you change this, you open a whole new can of worms.

Yellow timing is only part of the solution, speed limit, intersection size, flow volumes and other factors are also major players in determining how an intersection is timed. However as mentioned before, changing the yellow timing may in fact make things worse and isn't always possible due to flow volumes and signal timing. In order to keep traffic flowing, it isn't always possible or desirable to have a full 4 second yellow because that is time taking away from green time.

For more information check out:
http://mutcd.fhwa.dot.gov/

Traffic & Highway Engineering Third Edition
Nicholas J Garber and Lester A. Hoel
ISBN-13: 978-0-534-38743-3
ISBN-10: 0-534-38743-8

More information on the sensors being implemented in Denver can be found here:
http://www.redflex.com/html/usa/brochures/REDfixed.pdf

Drive Safe!

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Showing 1 of 1 comments

Submitted By: Michael Rule

posted on 5/24/2008 @ 6:43:32 AM

Rated Story

Good info, and good food for thought. The math is a bit taxing at 6:45 while I sip my decaf, though....

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Brad Bettag

Lakewood , CO

Brad Bettag has posted 40 stories and 46 comments since joining on 3/22/2006. Brad Bettag 's average story rating is 3.72.

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