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Status Report, Vol. 36, No. 4 | April 28, 2001 Subscribe

Small changes in signal timing could reduce crashes

More than 1 million crashes occur annually at U.S. intersections with traffic signals. To reduce these, officials in many cities are using red light cameras to reduce deliberate signal violations. New research indicates that retiming yellow signal lights also would reduce crashes.

The yellow light phase plus the very short all-red phase that follows the yellow light at many intersections is known as a change interval. Research has established that the duration of this interval can affect the chance of inadvertent red light running. Based in part on research conducted by the Institute during the 1980s (see "Intersections with poor timing have more vehicle crashes," July 14, 1984), the Institute of Transportation Engineers (ITE) adopted a proposal for establishing the duration of traffic signal change intervals. A recent Institute study shows the recommendations are effective.

In particular, modifying change intervals in accordance with ITE's proposed recommendations can reduce the risk of crashes involving pedestrians and bicyclists. Such modifications also may reduce the overall risk of multiple-vehicle crashes, particularly crashes involving injuries.

Working with state and local transportation officials in New York, Institute researchers identified 122 standard four-leg intersections on Long Island. For the study, roughly half of these intersections were designated as controls and half were considered for signal retiming. Application of the ITE formulas indicated that the duration of the change intervals should be modified at 40 of the 51 experimental sites.

Eight percent fewer reportable crashes — that is, crashes with injuries or $1,000+ damage — were recorded during the 36 months after the signal lights were retimed, relative to the control sites. Crashes involving pedestrians and bicyclists dropped 37 percent relative to the control group of intersections. The researchers found a 12 percent reduction in injury crashes. The experimental sites were 9 percent less likely than the controls to report multiple-vehicle injury crashes.

"Such benefits suggest that many urban intersections could benefit from similar modifications," says Richard Retting, the Institute's senior transportation engineer.

The ITE formula computes the yellow interval timing as a function of approach speed and grade, along with assumed values for perception-reaction time, deceleration rate, and acceleration due to gravity.

Crash reductions after signal light timing changes, compared with control sites

-8% overall
-12% injury crashes
-37% pedestrian and bicycle crashes

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