Travel time reliability

Summary

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According to FHWA, travel time reliability measures the extent of this unexpected delay. A formal definition for travel time reliability is: the consistency or dependability in travel times, as measured from day-to-day and/or across different times of the day^[1]. In addition, there are many different ways to define travel time reliability. For example, according to NZTA, trip time reliability is measured by the unpredictable variations in journey times, which are experiencedfor a journey undertaken at broadly the same time every day. The impact is related to the day-to-dayvariations in traffic congestion, typically as a result of day-to-day variations in flow. This is distinctfrom variations in individual journey times, which occur within a particular period^[2]. As reviewed by Taylor (2013)^[3], there are many different concerns to consider when defining travel time reliability.

Travel time reliability has been increasingly recognized as a key performance indicator for transportation roadways and transport systems, which exerts a strong influence on the stakeholders in transportation networks, including users (travelers), service providers, planners, and managers. This has stimulated research into the development of measures to quantify the level of reliability or the extent of variability in travel times. As a result, several travel time reliability measures have been introduced over the last two decades. The reliability measures can be divided into three classes: (1) point-based measures, including probability-based, moment-based, percentile-based, tail-based, and utility-based measures, (2) bound-based measures, and (3) PDF-based measures.^[4]

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