Modifying Signal Timing During Inclement Weather

Most individuals living in cold climates realize that on snowy days their commute will take longer. Although traffic volumes are often lower, the combination of reduced speeds and capacity causes severe congestion, particularly on signalized urban networks. Signal coordination that reduces traffic congestion in typical clear conditions results in an uncoordinated and suboptimal timing plan. Traffic parameters for developing signal timings during inclement weather conditions are examined. With the completion of the Utah Department of Transportation (UDOT) advanced transportation management system, there is an opportunity to change signal timing plans by communicating with each controller from the Transportation Operations Center. This operation makes feasible a library of special signal timing plans, with one allocated for inclement weather. Traffic flow data were collected over a range of seven inclement weather severity conditions at two intersections for the 1999–2000 winter season. The data indicate that the largest decrease in vehicle performance occurs when snow and slush begin to accumulate on the road surface. Saturation flows decrease by 20 percent, speeds decrease by 30 percent, and start-up lost times increase by 23 percent. UDOT is now developing and implementing modified inclement weather coordinated signal timing plans for the major signalized corridors in the Salt Lake Valley. The determination of when to implement an inclement weather signal timing plan is based on four general criteria: storm severity, projected duration, area of influence, and immediately projected running speeds. With these considerations, traffic engineers can determine whether to implement an inclement weather signal timing plan.