Incorporation of travel time reliability into the HCM. SHRP 2 Reliability Project L08, prepublication draft, not edited.

The objectives of SHRP 2 Project L08 are twofold. The first objective is to incorporate non-recurring congestion impacts into the HCM procedure. The second objective is to expand the analysis horizon from a single study period (typically an a.m. or p.m. peak period) to an extended time horizon of several weeks or months to assess the variability and the quality of service the facility provides to its users. Travel time reliability aims to quantify the variation of travel time. It is defined using the entire range of travel times for a given trip, for a selected time period (for example, the pm peak period on weekdays), and over a selected horizon (for example, a year). A deterministic approach to scenario generation is proposed for freeway facilities. This deterministic approach enumerates different operational conditions of a freeway facility based on different realization of factors which affect the travel time. The scenario generation for urban streets consists of four sequential procedures. Each procedure processes the set of analysis periods in chronologic order. • The first procedure predicts weather event date, time, type (i.e., rain or snow), and duration. • The second procedure identifies the appropriate traffic volume adjustment factors for each date and time during the reliability reporting period.• The third procedure predicts incident event date, time, and duration. It also determines incident event type (i.e., crash or non-crash), severity level, and location on the facility. • The fourth procedure uses the results from the preceding three procedures to develop one urban streets engine input file for each scenario in the reliability reporting period. Because travel time reliability is a new concept for the transportation profession, the research team recommends that travel time reliability be used as a performance measure to describe travel characteristics on freeways and urban streets. Subsequently, consideration can be given to using travel time reliability to define level of service. (Author/publisher)