This study uses planning time index 80 (PTI80) as the measure of travel time reliability. It is defined asPTI80=T80T0(6)where T80 is the 80th percentile travel time and T0 is the free-flow travel time. A PTI80 value of 1.5 means that for a 30-min trip in light traffic, 45 min should be planned to ensure on-time arrival 80% of the time.

PTI80 can be calculated directly using measured travel times, or estimated as a function of the travel time index (TTI) (46), which is the ratio between the average travel time and the free-flow travel time. The estimated relationship for each observation i takes the formPTI80i=γ1TTIiγ2(7)where γ1 and γ2 are estimated model parameters. These parameters were estimated for this study from the observed travel time data from both 2010 and 2016, with one observation for each TMC, TOD, and year combination. The relationships are specific to each facility type. Table 1 shows the results of that estimation. PTI80 was calculated for each TMC link, TOD, and year combination, and was aggregated to the network level using a VMT-weighted average.

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