The Urban Congestion Report (UCR): Documentation and Definitions
What Has Changed in the UCR?
Traditionally, the UCR has been developed using archived traffic operations data from roadway sensors. The biggest change is that the UCR is now developed (starting with the October 2014-December 2014 report) using vehicle-probe-based travel times. There are a number of other updates in the UCR, including
- the number of cities being reported (from 23 to 52),
- the definition of city boundaries using metropolitan statistical areas (MSAs),
- more complete roadway coverage within the cities, and
- use of Highway Performance Monitoring System (HPMS) database traffic counts for vehicle miles traveled (VMT) weighting.
How Are the Measures Developed?
Congested Hours are computed as the average number of hours during specified time periods in which road sections are congested — speeds less than 90 percent of free-flow speed (e.g., 54 mph if free-flow speed is 60 mph). This measure is reported for weekdays (6:00 a.m. to 10:00 p.m.). Averages are weighted across road sections and urban areas by VMT using volumes from FHWA's HPMS.
The Travel Time Index is the ratio of the peak-period travel time as compared to the free-flow travel time. This measure is computed for the AM peak period (6:00 a.m. to 9:00 a.m.) and PM peak period (4:00 p.m. to 7:00 p.m.) on weekdays. Averages across urban areas, road sections, and time periods are weighted by VMT using volume estimates derived from FHWA's HPMS.
The Planning Time Index is the ratio of the 95th percentile travel time as compared to the free-flow travel time. The measure is computed during the AM and PM peak periods as defined in the TTI, and averages across urban areas, road sections, and time periods are weighted by VMT using volume estimates derived from FHWA's HPMS.
The free-flow speed is the 85th percentile of off-peak speeds, where off-peak is defined as Monday-Friday 9a-4p and 7p-10p, as well as Sat-Sun 6a-10p. For the purposes of UCR, the free-flow speeds are calculated for each TMC path and are based on the previous 12 months of data.
What data is used to calculate the measures?
In an effort to use the best data available for the FHWA UCR program, FHWA's Office of Operations acquired the National Performance Management Research Data Set (NPMRDS). This historical traffic speed data set covers the entire National Highway System (NHS). It includes observed measurements (collected 24 hours a day) and provides the user with average travel times in 5-minute intervals in three ways — freight truck, passenger vehicles, and all vehicles. FHWA has made the data set available to State Departments of Transportation and Metropolitan Planning Organizations for use in their performance management activities. The data are available monthly.
How Are the Data Processed?
The following highlights key calculation procedures of the NPMRDS-based UCR:
- The basic spatial unit of analysis for the NPMRDS-based UCR is traffic message channel (TMC) paths, which are relatively short (an average of 1.3 miles among all 52 MSAs) directional roadway segments that are defined by a consortium of commercial traffic information providers.
- As a first step in the performance measure calculation, researchers summarized the 5-minute day-by-day travel times into 15-minute monthly average travel times by day of week (e.g., each TMC path should have a travel time value for 6:00 to 6:15 a.m. for Mondays in January, 6:00 to 6:15 a.m. for Tuesdays in January, etc.).
- To make meaningful year-to-year comparisons in the NPMRDS-based UCR, it is necessary to keep certain calculation parameters as consistent as possible; therefore, researchers used a 12-month trailing average (of monthly free-flow speeds) for performance measure calculations.