Rationale for Incorporating Queue Discharge Flow into Highway Capacity Manual Procedure for Analysis of Freeway Facilities

Abstract

The freeway facilities methodology in the 2010 Highway Capacity Manual (HCM) is the only HCM methodology that encompasses undersaturated and congested flow regimes over multiple periods. However, the methodology is limited by its assumption of a fixed capacity threshold between the two flow regimes. The method does not consider the two-capacity phenomenon, which suggests that a drop in the throughput from theoretical capacity is observed after breakdown has occurred. A summary of the available literature on empirical evidence of the capacity drop under queue discharge conditions offers a theoretical evaluation of the impact on queue discharge flow based on shock wave theory relationships that form the basis of the queuing model used in the HCM freeway facilities method. Examples illustrate incorporating queue discharge into a freeway facilities analysis, and implications for practice are discussed.

Innovative Work Zone Capacity Models from Nationwide Field and Archival Sources

“…When comparing average QDR and PBC, the authors identified an average of 13.4% capacity reduction caused by breakdowns found in freeway work zones, on the basis of the literature data. The value was twice as much as reported for freeways without work zones (23).…”

“…Although it is not adopted in the current HCM, the "two capacity phenomenon" that distinguished QDR and PBC was well represented throughout the literature. Specifically, an average decrease of 5% to 7% of QDR compared with PBC under non-work zone conditions was reported in a previous meta-analysis of various sources (23). When volume and speed data were collected together, some studies used existing speed-flow models to calculate a single capacity value (5-8, 12, 13).…”

Incorporating Travel Time Reliability into the Highway Capacity Manual

“…This feature was not provided in the HCM2010. However, Hu et al (2012) found that it has a significant effect on the duration and severity of the congested region. That study found that, according to an extensive literature search, capacity loss averaged 7% during breakdown.…”

Modeling Traffic Incident Duration Using Quantile Regression

“…Traffic incidents occur frequently on roadways, resulting in congestion, commuter anxiety, and harmful vehicular emissions (1)(2)(3). One traffic incident management strategy is to disseminate accurate incident duration information to travelers (e.g., through variable message signs), who can then make more informed travel decisions (4,5).…”