Review of the HCM6 Capacity Estimation Methodology for Freeways

Abstract

Chapter 26 of the Highway Capacity Manual sixth edition (HCM6) suggests a procedure for the empirical estimation of freeway capacity, which is based on the direct estimation of breakdown probabilities for bins of traffic volumes. This paper expounds that this methodology is unsuitable for obtaining reliable capacity estimations. The theoretical analysis of the deficiencies of the methodology is supported by empirical capacity estimations for 12 freeway sections in California. Based on the empirical results, alternatives to the HCM6 capacity estimation methodology including statistical models for censored data as well as the distribution of prebreakdown volumes are proposed and validated.