Impact of Adverse Weather on Freeway Bottleneck Performance

Congestion on freeways occurs when demand exceeds the available capacity. Common causes of recurring congestion, also known as freeway bottlenecks, include lane drops, on-ramp merges, and weaving sections. Adverse weather can reduce the maximum queue discharge flow, but this effect has not been systematically investigated. This research examined the relationship between discharge flow and weather characteristics including rainfall intensity, wind speed, and visibility. Queue discharge rates at four isolated merge bottlenecks were measured using an established methodology of cumulative count and occupancy curves. An analysis of discharge variation by rainfall intensity revealed reduced discharge ranging from 5% in drizzle (rainfall $<0\mathrm{in.}/\mathrm{h}$) to 27% in heavy rainfall [rainfall $>2.54\mathrm{m}\mathrm{m}/\mathrm{h}$ ($>0.1\mathrm{in.}/\mathrm{h}$)]. However, rain intensity accounts for only a portion of the variability in discharge flow. Two hypotheses were tested using the additional variables of wind speed and visibility as well as dividing the periods of discharge flow into multiple groupings. Analyses based on these hypotheses described the variation in queue discharge flow better than the analysis with a single independent variable. This research showed that weather characteristics are an important predictor of bottleneck queue discharge rates.