Using Travel Time Reliability Measures to Improve Regional Transportation Planning and Operations

Travel time estimation is of increasing importance to travelers and transportation professionals alike as congestion worsens in major urban areas. In fact, the reliability of travel time estimates on a given corridor may be more important for travelers, shippers, and transport managers than the travel time itself. This paper examines the uses of measured travel time reliability indices for improving real-time transportation management and traveler information using archived ITS data. Beginning with a literature review of travel time reliability and its value as a congestion measure, a content analysis of twenty regional transportation plans from across the nation is then described. Results from the content analysis indicate that travel time reliability is not currently used as a congestion measure, and that the most common measures of congestion were the volume-to-capacity ratio, vehicle hours of delay, and mean speed. Then, as a case study using data from Portland, Oregon, several reliability measures are tested including travel time, 95th percentile travel time, travel time index, buffer index, planning time index, and congestion frequency. The buffer index is used to prioritize freeway corridors according to travel time reliability. Metropolitan planning organizations should use travel time reliability in the following ways: 1) incorporate it as a system-wide goal; 2) evaluate roadway segments according to travel time reliability measures; and 3) prioritize roadway segments using those measures. INTRODUCTION Travel time reliability is defined as the consistency of a given trip’s travel time. More formally, it is said to be “the consistency or dependability in travel times, as measured from day to day and/or across different times of the day.” (1) It is possible to consider reliability in the historical sense such that a distribution of travel times is examined, and specific statistics can be reported, such as the mean, median, standard deviation, variance, etc. Thus, the degree of historical variability of travel times can be reported as the reliability of a particular repeated trip. On the other hand, reliability in a real-time sense can be considered, where a trip being taken now is compared to some sort of pre-set standard. It is clear that reliability is an important measure of the health of the transportation system in a region. Individual travelers and industries depend on the transportation system, and reliability can make a significant difference in efficient use of the transportation system. Despite its importance, transportation planners have not begun to include reliability as a performance measure. Instead, planners typically use other measures of congestion as well as measures of accessibility, mobility, connectivity, and safety to evaluate the transportation system’s health and determine where future investments are needed. This paper will describe how measures of congestion are used in that evaluation and will suggest that adding travel time reliability measures to the ones currently in use can result in a better understanding of the transportation system. A content analysis performed on a random sample of regional transportation plans (RTPs) from across the nation focused on uses of reliability within each plan, and revealed that reliability measures are not currently in use. With this in mind, this paper will explore ways in which travel time reliability is measured using archived data over a segment, a corridor and an entire urban area. It is important to communicate the ways that transportation investments can benefit an entire region. Increased amounts of information on travel time reliability can better inform travelers of the most appropriate usage of the system for their purposes. Using data from 2004–2007, travel time reliability measures are demonstrated and described in the context of the Portland, Oregon metropolitan region. The analysis shows changes in travel time reliability over time, and a case study compares rankings of instrumented Portland freeways according to their travel time reliability ratings versus more traditional congestion measures.