Proposed Chapters for Incorporating Travel Time Reliability into the Highway Capacity Manual

Abstract

This document contains two proposed chapters for the Highway Capacity Manual (HCM) that introduce the concept of travel time reliability and offer new analytic methods. Proposed Chapters 36 and 37 set out methodologies for incorporating reliability into the HCM analytic procedures for freeway facilities and urban streets. The approach is to generate many freeway and urban street scenarios involving various causes of nonrecurring congestion, such as incidents, weather, and work zones, and use the scenarios as input to a computational engine to calculate travel time over a segment. The travel times for each scenario are used to construct a distribution of travel time from which reliability performance measures can be derived. Chapter 37 supplements Chapter 36. It provides reliability values for selected U.S. facilities, offers an alternative freeway incident prediction method, elaborates on the freeway and urban street scenario generators, explains how to measure reliability in the field, and gives an example problem. The chapters were prepared under the Transportation Research Board’s second Strategic Highway Research Program (SHRP 2) Project L08, Incorporation of Travel Time Reliability into the Highway Capacity Manual, but they have not been officially accepted by the Highway Capacity and Quality of Service (HCQS) Committee of the Transportation Research Board.