This document contains two proposed chapters for the Transportation Research Board’s Highway Capacity Manual (HCM) that introduce the concept of travel time reliability and offer new analytic methods. The chapters were prepared under 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. The HCQS Committee has responsibility for approving the content of the HCM. The scope of work for SHRP 2 Reliability Project L08 called for revising the methodologies for freeway facilities and urban streets. This research has resulted in a prospective Chapter 36 for the HCM concerning freeway facilities and urban streets and a prospective supplemental Chapter 37 that elaborates on the methodologies and provides an example calculation. In addition, a report documenting the research effort was prepared. It includes the user’s guides for the computational engines for freeways and urban streets. As with all SHRP 2 research, it is standard procedure to publish the key documents that result from each research project. The National Academies have approved Chapters 36 and 37 and the final report for publication as SHRP 2 products. The HCQS Committee, responsible for approval of changes to the HCM, has begun considering this material. 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. (Author/publisher)
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