Access to Destinations: Computation of Travel Time Data for Access to Destinations Study

Freeway travel time data is used as an important input for measuring travel reliability and accessibility. The goal of this project was to generate reliable travel time data using loop detector data from the Twin Cities' freeway network collected over the past 14 years for the Access to Destinations projects. Several difficulties exist: one year of loop data can be missing up to 31.7%, loop data does not include vehicle lengths or classification to use in speed calculations, and link travel time must be computed without knowing speed variability within the link. The three basic approaches used in this project for imputing speed data are linear regression, spatial imputation, and week-to-week temporal imputation. A new method was developed to estimate average vehicle length using volume, occupancy, free-flow speed, and speed limit data. Link travel times were calculated by dividing the distance between stations into thirds and using speed data from both stations. Overall (for the last fourteen years), the imputation increased the average amount of valid data from 81.7% to 98.6%. Travel time for a selected route was verified by comparing the resulting calculated travel times to manual travel times measurement information.