Flow Breakdown at Freeway Bottlenecks

Abstract

An automated analysis of cumulative flow and speed curves was used to investigate issues related to flow breakdown at freeway bottlenecks, including ( a) the relationship between prebreakdown flow rates and the amount of time they can be sustained, ( b) flow trends in the period immediately before breakdown, and ( c) the probability that flow decreases at breakdown. The automated analysis allowed consistent application of explicit rules to identify bottleneck activation and deactivation, periods of nearly constant flow, and so on. Data analyzed did not provide evidence of a strong relationship between the flow rate and the amount of time that nearly constant prebreakdown flow can be sustained, although a weak relationship may be present. Periods of nearly constant flow occurred before initial flow breakdown about half the time; the average duration of these periods was roughly 30 min. Periods of both increasing and decreasing flow were also observed. Flow decreased at breakdown roughly two-thirds of the time. The lack of a strong relationship between the flow rate and time to breakdown suggests that ramp metering is unlikely to be effective in prolonging high-volume prebreakdown flow. Metering is more likely to be effective if it is used to eliminate or control very specific causes of flow breakdown, such as arrival of dense platoons of ramp vehicles at the merge point or high concentrations of traffic in the right lane immediately downstream from a ramp.