Impacts of Signal System Timings on Rain Related Congestion

It is known that inclement weather can affect traffic volumes, vehicle speeds, speed variance, saturation flow rates, and sometimes discharge rates from traffic signals. These parameters in turn can have a significant impact on the efficiency of traffic signal timing plans. Timing plans developed for dry conditions with good visibility may not be as effective under wet conditions with poor visibility, even for the same traffic volumes. Previous studies have looked at the effects of poor weather on traffic signal efficiency but they have focused primarily on winter weather conditions involving snow and ice, which have much more dramatic impacts on traffic flow. In regions like the South, rain is the primary adverse weather event and even though its effects on traffic flow may be smaller than snow or ice, its impacts can still be seen. This study sought to assess the impact of rain on traffic flow and determine whether there can be a meaningful benefit to developing and implementing signal timing plans specifically for rain events. Using simulations for two study corridors in Birmingham, Alabama, it was found that moderate and heavy rain events do impact traffic flow, increasing stops and average delays up to 6% compared to dry conditions. Signal timing plans optimized for wet conditions were found to reduce these delays, though in absolute terms the benefits were modest. It was found, however, that the effectiveness of rain-specific timing plans depended to a large extent on prevailing traffic volumes. An evaluation of potential costs and benefits resulting from rain-optimized signal timing plans found that the net benefits, at least in the two study corridors, are questionable. The analysis found that projected costs over a five-year life cycle would significantly exceed potential benefits. Rain-specific timing plans may have useful applications in certain corridors but the potential benefits and costs should be carefully evaluated beforehand.