Citation

Dehman A. Transp. Res. Rec. 2012; 2286: 84-93.

Copyright

(Copyright © 2012, Transportation Research Board, National Research Council, National Academy of Sciences USA, Publisher SAGE Publishing)

DOI

10.3141/2286-10

PMID

unavailable

Abstract

Inclement weather conditions are well known to reduce the capacity of freeway sections. Recent trends distinguish between definitions of two capacity regimes: free-flow capacity, defined by the prebreakdown flow (PBDF), and congested-flow capacity, defined by the queue discharge flow (QDF). The weather effect on PBDF and QDF is investigated, and weather reduction factors are proposed for use in estimating freeway bottleneck capacity for both the free-flow and congested-flow regimes. The behavior of four identified bottlenecks in the Milwaukee, Wisconsin, freeway system was analyzed under different weather conditions. More than 1,300 breakdown events were included in the analysis. Effects of rain, snow, and fog were all evaluated, with special consideration for the degree of visibility and precipitation intensity, and it was found that QDF is more sensitive to weather conditions than is PBDF. Categorizing weather variables by precipitation and visibility allowed for the development of more-accurate capacity reduction factors. Five-minute PBDF inclement weather reduction factors were statistically significantly larger than 15-min PBDF corresponding factors; however, differences between the two were very small. The refined QDF reduction factors are not only applicable at recurrent weekday freeway bottlenecks, but also may find applications at work zones and other types of bottlenecks where traffic congestion prevails.