Potential Effects of Wet Conditions on Signalized Intersection LOS

Although rain and other precipitation is common, analyses of signalized intersections based on the Highway Capacity Manual require clear weather and dry pavement conditions. Three factors may be affected by wet weather: saturation flow, effective green time, and progression. Saturation headway may lengthen, effective green may shrink, and progression may worsen in wet conditions. As a result, signalized intersection operations become less efficient. This is demonstrated in the longer delays estimated for five sample intersections. A methodology was developed for the derivation of the probability of rainfall $\left(P_{\text{wet}}\right)$ during the morning and evening peak hours for any intersection in the United States using readily available rainfall accumulation data from the National Oceanic and Atmospheric Administration. Once delay and level of service (LOS) are estimated for both dry and wet conditions, a weighted average is employed for estimating delays and LOS that represent the prevailing dry and wet peak period conditions at a signalized intersection.