Reliability Breakpoints on Freeways

Abstract

This paper shows through the evaluation of probe vehicle and loop detector data that travel time reliability on a freeway is NOT a function of counted traffic volumes until a “breakpoint volume” is reached. At that breakpoint the travel time reliability decreases (variance increases) abruptly. Once the breakpoint volume is exceeded, the decrease in travel time reliability is so extreme and abrupt as to suggest It is a vertical function, with a non-singular relationship to further volume increases. The breakpoint in reliability generally occurs at a counted volume significantly lower than the theoretical capacity of the facility computed per the Highway Capacity Manual. The breakpoint volume varies significantly between facilities and within the same freeway facility (by location and direction of travel on the same facility). The breakpoint volume does not appear to be a fixed ratio of the theoretical capacity of the facility. The study is based on a year’s worth of 5 minute speed and volume counts at 4 detector stations located on two freeway facilities in the San Francisco Bay Area. Three weeks of 1-minute probe vehicle travel times were also evaluated for one of the facilities and compared to the detector station estimates of travel time reliability. However, it is recognized that significant additional data from other locations around the country is needed to better evaluate the factors affecting travel time reliability.

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