Conspiracy Theory

Several times a year, the phone rings at the office and someone on the other end has questions for me about the length of the yellow interval for traffic signals.  The call usually goes something like this:

Reporter:  “Hi.  This is Kent Brockman from WXYZ in Shelbyville.  We’re doing a story on the traffic signal timings and were wondering if you could answer some questions for us.”

Me:  “Sure, I’d be happy to help.  What are your questions?”

Report:  “I took a stop watch out to some intersections and think the yellow interval is too short.  I calculated what the interval should be based on the traffic engineering department’s equation, and I get a much longer time.”

Me:  “Okay, let me walk through the calculation with you.  Tell me about the intersection.”

These calls are usually initiated because it’s a slow news day, the local police department announced the start of automated enforcement, or the reporter’s spouse got a ticket for running a red light.  Regardless of the reason, I’m usually glad to help.  Most reporters are very congenial and have made a simple mistake in the calculation.  Although appreciative, some are disappointed after I walk through the calculation with them only to find that the yellow interval at the signal is adequate.  These conversations happen so regularly that I keep a three-ring binder with the derivation of the equations and associated pictures that I fax or email out when needed. 

I also get a few calls a year that are similar, but go a little more like this:

Caller:  “Hi.  This is Lionel Hutz from Springfield.  I’m a traffic safety expert.  Someone gave me your name.  I have some questions about the yellow interval.”

Me:  “Sure, I’d be happy to help.  What are your questions?”

Caller:  “I think there is an error in the yellow interval equation.” 

Me:  “I’d like to hear a little more about why you think there is an error.  But first, how did you get my name?”

Caller:  “The interval is too short.  The calculation has a math error.”

Me:  “Okay…. I don’t think there is a math error but we’ll walk through it together.   Let’s start with the equations of motion and assume we have two cars approaching a signal side by side when the light turns yellow.  One car decides to stop; the other car decides to keep going.  We’re going to set up two equations.  One equation is based on the distance…”

Caller [Interrupting]:  “I’ve looked at that.  There is not enough time for the stopping car to stop.  There is a mistake in the equation.”

At this point, I realize the caller misspoke at the start of the conversation.  When they said they were a traffic safety expert what they really meant to say was ambulance chaser.  However, I usually still try to help, hoping to change a misperception. 

My expertise in the yellow interval started in 2001 after the staff of the U.S. House Majority leader at the time released a report entitled, The Red Light Running Crisis:  Is it Intentional?  The title might lead you to think the report was questioning whether drivers were intentionally violating the signal but instead, the report questioned whether traffic engineers had spent twenty years changing the equation to calculate the yellow interval in order to make money on red light cameras. 

Although I love a good conspiracy theory, two months of research at the Library of Congress unearthed that the equation used by most agencies (referred to as the kinematic equation) was based solidly on the equations of motion and the best available research about driver’s reactions and vehicle capabilities.  The inputs to the equation have changed through the years in response to updates in research from what was available in the 1920s.   For example, one of the largest changes happened about 20 years ago when the rate at which vehicles stopping were assumed to decelerate was lowered.  (This changes causes a lot of raised eyebrows but actually resulted in longer intervals because vehicles that decelerate slower need a longer distance.)

Based on our work, Hugh McGee and I wrote the congressional testimony to address the allegations in the report.  This testimony was eventually published as “A History of the Yellow and All-Red Interval.”  With the help of Google, this publication is usually how reporters get my name. 

The history of the yellow interval is about to add another chapter.  In a few months, the results of a National Cooperative Highway Research Program project (NCHRP 3-95) on the subject will be published.  Hugh McGee is the Principal Investigator of the study which developed a comprehensive and uniform set of recommended guidelines for determining safe and efficient yellow change and red clearance (i.e., all-red) intervals at signalized intersections.  The guidelines are insightfully based on lots and lots and lots of observations of drivers approaching signals at the onset of the yellow interval around the U.S. 

At the TRB Annual Meeting this year, you can get an advance look at the findings in two presentations.  My friend and Hugh’s teammate on this project, Dr. Tim Gates from Wayne State University, will present the findings of the field studies on perception-reaction, deceleration rate, and approach speed in the presentation A Comprehensive Evaluation of Driver Behavior to Establish Parameters for Timing of Yellow Change and Red Clearance Intervals.  In a second presentation, my colleague and friend at VHB, Kevin Moriarty, will present the guidelines in Guidelines for Timing Yellow and Red Intervals at Signalized Intersections.

This promises to be one of the best sessions at TRB, although if you’re hoping for a conspiracy, you’ll have to look elsewhere.