Travel time reliability is the ability of travelers to reach their destination in a consistent and dependable amount of time. A highway’s travel time may be reliable and still experience slowdowns, but these slowdowns will be predictable and recurring (rush hour for example).

Bottlenecks, weather, crashes, special events, and other factors can decrease reliability. The level of travel time reliability is measured by the ratio of the 80th percentile travel time to the normal (50th percentile) travel time throughout a full calendar year. Ratios of greater than 1.5 are considered unreliable.