Reliability, specifically travel time reliability, quantifies the variation of travel time. Per the HCM, reliability is defined as “the entire range of travel times for a given trip for a selected time period and over a selected horizon.” Reliability is expressed in terms of variability or in terms of success or failure with respect to a specific target. To measure reliability, a history of travel times over a selected horizon (for example, a year) is necessary to create a travel time distribution.