8/13/2015Traffic Signal Pioneer Warns Of Overly Short Yellow Lights
Original co-author of yellow signal timing formula tells engineering body it has misinterpreted his work.
Transportation officials gathered in Hollywood, Florida last week for an Institute of Transportation Engineers (ITE) conference. They were surprised to learn that one of their leading members says they are doing signal timing wrong. Alexei Maradudin, an original author of the ITE yellow signal timing formula first developed in 1959, wrote a letter urging the organization to reform the most recent revision of its signal timing procedures.
"In reviewing the proposed recommended practice document, I am concerned that some of or work has been taken out of context, which will result in the kinematic formula being misapplied to situations were it cannot, by its very nature, be applied," Maradudin wrote to ITE leaders.
For more than a decade, critics have charged that ITE has tinkered with the signal timing formula in ways meant to facilitate the adoption of red light camera enforcement. In 2001, the majority leader of the US House of Representatives issued a report blaming the ITE formula for creating a "red light running crisis" (view report) with inadequate yellow warning times. Now Maradudin, a physics professor at the University of California, Irvine says the abuse of his kinematic formula is short-changing motorists, particularly at left-turn intersections.
"This formula, which we derived, cannot be applied to turning lanes or to any situation where the driver must decelerate within the critical distance," Maradudin wrote. "Any method chosen to determine the proper yellow time for turning lanes must account for the driver to slow down in order to make the turn safely."
While the new ITE proposal encourages slightly longer yellows for drivers who are proceeding straight through an intersection, those making either right or left-hand turns face shortened yellows. The proposal allows local jurisdictions to set yellow turn lanes at 5 MPH below the posted speed limit, which is often set far below the prevailing speed of traffic. This is a mistake, Maradudin insisted.
"Our methods required that a motorist driving along a road within the legal speed limit be provided with a solvable solution to the stop or go problem encountered when the amber signal is illuminated," Maradudin wrote. "Setting a yellow interval based on an approach speed lower than the legal speed limit does not fulfill this requirement. Any driver approaching an intersection within the legal speed limit, but a speed higher than the assumed approach speed (in your recommendations the posted speed limit minus 5 MPH), may encounter dilemma zone and have no solvable solution to the stop or go problem."
Joe Bahen, an ITE member and licensed engineer for the National Motorists Association, agrees that the turning formula must be changed.
"We have repeatedly pointed out that there is no scientific basis for the 'speed limit minus five' estimate for highways where the speed limit is 35 MPH or less," Bahen explained. "Red light camera companies need only use Google Maps and look for long left-turn pockets to identify approaches where cameras would be extremely profitable."
A fraction of a second difference in yellow time can have a significant influence on the number of red light camera citations issued. The majority of straight-through red light "violations" happen when a driver misjudges the end of the yellow light by less than 0.25 seconds -- literally the blink of an eye (view Texas Transportation Institute (TTI) chart). In most cases, a yellow shortened by one second can increase the number of tickets issued by 110 percent, according to TTI (view report).
A copy of Maradudin's letter is available in a 250k PDF file at the source link below.