SELECTING TRAVEL RELIABILITY MEASURES

Reliability and variability in transportation are being discussed for a variety of reasons. The two terms are related, but different in their focus, how they are measured, how they are communicated and, in some respects, how they might frame the discussion about potential solutions. Reliability is commonly used in reference to the level of consistency in transportation service for a mode, trip, route or corridor for a time period. Typically, is viewed by travelers in relation to their experience. Variability might be thought of as the amount of inconsistency in operating conditions. This definition takes more of a facility perspective and, therefore, relates to the concerns of transportation agencies. Both of these concepts are useful, but the term may have a more marketable connotation for the purposes of reporting performance measures to the public because it relates to an outcome of transportation--the quality of the service provided. Variability seems to be more related to the change in transportation system operations. The traveling public and a variety of companies or product sectors use the term reliability in their goal statements and it would seem this is the term that should be used with a performance measure. This report discusses the background for developing measures as a component of mobility performance metrics, the factors to consider before selecting a measure and defines the calculation procedures for typical analyses. This might involve a change in thinking about how mobility service is conceptualized and communicated. The terms recurring and non-recurring congestion are misnomers--variations in weather, traffic volume and a range of other factors can be termed recurring but have traditionally been included outside the regular congestion label.