Some Properties of Flows at Freeway Bottlenecks

The capacity of a freeway segment should be measured only when it is an active bottleneck. The properties of flows at active freeway bottlenecks have a bearing on both the definition of capacity and the procedure of capacity analysis. Past studies have examined the flow features at bottlenecks on several freeways in Toronto, Canada, and San Diego, California. This study examined 27 active bottlenecks in the Twin Cities metro area in Minnesota for a 7-week period. The analysis focuses on the properties of prequeue transition flows (PQFs) and queue discharge flows (QDFs) averaged across various time intervals (30-s, daily average , and long-run average). It is found that the proportion by which flows drop after upstream queues form at all studied bottlenecks ranges from 2% to 11%. The 30-s QDFs display high variation and should not be assumed to be constant. The daily average QDFs at each studied bottleneck follow a normal distribution based on two normality tests and visual inspection of the normal probability plot. Results also suggest that the long-run average QDFs [mean of 2,016 passenger cars per lane per hour (pcplph)] and PQFs (mean of 2,124 pcplph) are both normally distributed. The implication of these empirical findings on capacity estimation is also discussed. The Highway Capacity Manual (HCM) (1) defines capacity as " the maximum hourly rate at which persons or vehicles reasonably can be expected to traverse a point or a uniform section of a lane or roadway during a given time period under prevailing roadway, traffic, and control conditions. " Later, in the glossary chapter, the definition differs slightly and the maximum sustainable flow rate is emphasized. The definition clearly states that the capacity of a freeway element should not be measured when the traffic condition is governed by a bottleneck further downstream. Therefore, capacity should be measured only at, or immediately downstream of, active bottlenecks , not within the queue upstream. Hall and Agyemang-Duah (2) address this issue in detail and conclude that studies examining capacity in the congested region of the fundamental diagram are " based on a mistaken premise about where the data are collected. " Therefore, capacity is a parameter dependent only on the flow features at active bottlenecks. However, current knowledge about the flow features at active bottlenecks suggests that the HCM does not provide a satisfactory method of operationalizing the definition of capacity. Terms appearing in the definition of …