The Florida trial of baseball hero Jim Leyritz is expected to feature evidence from a red-light-control camera. These robo-traffic-cop systems have recently become controversial in this corner of the world (northern Illinois). One Chicago suburb, for example, installed very short traffic lights that caused shoppers at one of the nation’s largest shopping malls to trigger red-light violations at such a high rate that retailers in the area demanded the city shut the cameras off. They were driving shoppers away from the mall. In Chicago proper, drivers often claim that yellow-lights have been shortened so that more red-light violations are triggered.

These cameras are like speed traps in rural towns: the cops rig the system to boost traffic violation revenue, not to improve public safety.

The Leyritz Incident

I’m not clear on the details of the late-night accident in which Jim Leyritz’s car hit another car in a Fort Lauderdale intersection, but among the bits of information I heard on In Session recently was pre-trial testimony of an expert on traffic-enforcement cameras. Most of what she told the court involved the time stamps on the video frames.

But it seems to me that the length of the lights might also be relevant. If Chicago’s yellow lights were shortened when traffic cameras were installed, it might also be possible that Florida’s yellow lights have recently been shortened, too.

Obviously, experienced drivers are acclimated to yellow lights of a fixed duration. If some yellow lights are shorter than others, that would pose a hazard.

The city of Chicago has a very informative (and defensive) page about robo-traffic-cop lights. It explains that the system (in Chicago, at least) relies on two triggers: 1) a sensor in the pavement right in front of the white stop line and 2) the light turning red. The Chicago faq states:

The digital cameras are tied into the traffic signal system and sensors beneath the pavement, just before the white stop bar.  The cameras are triggered by a vehicle passing over the sensors only after the light turns red.  The cameras take still and video pictures of the rear of a vehicle, including the license plate.

The faq also states that Chicago’s yellow lights are 3 to 4 seconds long, depending on the speed limit of the street. Supposedly this yellow-light duration is “well within” the federal Department of Transportation guidelines. Nonetheless, the city has received complaints about this and received requests to lengthen yellow lights. The Chicago faq says:

Why not just make yellow light [sic] longer?

Changing signal timings will not solve the problem of drivers running red lights.

The purpose of the yellow light is to warn drivers that the light is turning red.  It is not intended to promote speeding or risk-taking.  Unfortunately, too many drivers believe that yellow is a sign to speed up -- when in reality it should be a sign to slow down.

Extending the yellows won't solve the problem because motorists will learn that they now have an additional second or two, and will still treat the yellow as an extension of the green.  The behavior that some drivers exhibit—running red lights—will not change.  Longer yellow lights will not be beneficial to the overall safety of the intersection.

Additionally, a timing change would have a negative impact on traffic flow throughout the city, increasing congestion and reducing travel times.

Of course, the issue isn’t really the length of yellow lights, it’s different lengths at different intersections and also allegedly shortened yellow lights when traffic cameras are installed.

BTW: The idea that longer yellow lights would increase congestion is ludicrous; red lights are what increase congestion in the city’s busiest intersections. Reducing the length of red lights might actually reduce congestion. Why? Imagine a street in the Loop during rush hour. Traffic is “wall to wall” in both directions. A long red light on one street permits more traffic to pile up in front of it, usually producing a line of cars longer than the city block along which it runs. The traffic comes to a dead stop. Cars (taxis, especially) enter the intersection on the yellow light and stop in the middle before the light turns red, blocking the intersection for the green-lighted traffic. That’s why Chicago stations traffic cops at all the intersections, to direct traffic and over-ride the traffic lights.

I believe such chop-logic is rampant in “traffic science”: When I recently requested a 4-way stop sign at a dangerous corner near my house, the village supposedly “studied” the traffic patterns and concluded that “as studies have proven” too many stop signs cause people to run stop signs. Hmm, no stop sign is dangerous but a stop sign is more dangerous.

How long should a yellow light be?

The Department of Transportation’s guidelines for yellow lights—not surprisingly—aren’t all that easy to locate or to interpret. I found a listserv discussion on the DOT website with the following information:

The ITE recommended practice (ITE Technical Committee 4A-16, "Recommended Practice: Determinging [sic] Vehicle Change Intervals," ITE Journal, May 1985) for determining the appropriate length of a yellow interval is:

y = t + (85th percentile speed)/(2a + 64.4g) where:

• t = driver reaction time in seconds (1.0 second is a commonly used value for this parameter)
• 85th percentile speed is in ft/s
• a = deceleration rate of vehicles, fps^2 (10 fps^2 is a commonly used value for this parameter)
• g = grade of approach, expressed as a decimal (ie 2% downgrade = -.02 64.4 = 2 times the acceleration due to gravity

The rule of thumb expressed in the previous post of .5 seconds for every 5 mph results in a value that is less than recommended for 85th percentile speeds of 35 mph or less and a value that is larger than recommend for 85th percentile speeds greater than 35 mph for intersections at level grades.

I’m not a lawyer and I’m even more not a mathematician. I doubt that the people who install traffic cameras are either. Try as I might, I could not parse this supposed equation.

The “rule of thumb” calculation the author disparages makes more sense. At 30 mph that equation would result in a 3-second yellow light, but the DOT commenter says that’s too short. If so, the city of Chicago’s claim that a 3 second light is within guidelines is incorrect.

Back to Baseball

My question is, how long was the yellow light that preceded the red light Jim Leyritz is accused of running?

If he was speeding, as well as DUI, he might have entered the intersection while the light was yellow, thinking he could beat it. This, of course, would weigh heavily against him—not that he entered the intersection on a yellow light, which is legal, but that he was speeding and drunk.

If he wasn’t speeding and if he was below the legal limit, he might have legally entered the intersection on the yellow light, and that would mean the other car was the one running the red light, not him. That might exonerate him.

But if the DOT can’t explain its recommendations for the length of yellow and red lights to the traffic department of Chicago, how will the Leyritz jury be able to understand the issue? I’m fairly certain that none of the lawyers in the Leyritz trial is any better at math than I am.