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Mobility Monitoring – Roadways – Performance Measures
OVERVIEW • REGIONAL TRENDS • ROADWAYS • INTERSECTIONS • TRANSIT
BICYCLE/PEDESTRIAN • HOV LANES • TRANSPORTATION DEMAND MANAGEMENT • PARK-AND-RIDE
Overview • Arterial Roadways • Crashes • Freeways • Performance Measures • Traffic Volumes
The Congestion Management Process (CMP) identifies congestion on monitored roadway segments by examining a combination of the following travel-time-based measures: average observed travel speed, speed index, and delay. These performance measures are calculated from travel time data collected at peak commute times in typical traffic conditions.
Average Observed Travel Speed
Travel speed is a typical measure of performance for a roadway segment. These data are presented for arterial and limited-access roadways via several maps available for download on the Arterial Roadways and Freeways pages. Level of service (LOS) for a roadway or highway segment is determined using average-speed data.
Travel Speed Index
The posted speed limit is one of the factors that influence travel speeds on roadways. Therefore, in order to complement the average observed travel speeds, a speed index is used to account for the speed limit factor. The speed index is simply a ratio that is calculated by dividing a roadway segment´s average observed travel speed by the posted speed limit for that roadway segment. The index helps to determine whether a low observed speed is caused by congested conditions or simply by a lower posted speed limit. Speed index data are presented for arterial roadways via several maps available for download on the Arterial Roadways page.
Delay
For purposes of CMP monitoring, delay is defined as the time a vehicle travels below 5 mph on a roadway segment (including time that the vehicle is stopped), as long as the speed has been lower than 5 mph for at least three consecutive seconds. The observed delay is closely related to “control delay” (for arterial roadways), which is the delay that occurs when a vehicle moves forward in a queue, a slow stop-and-go process.

Measuring delay is one aspect of evaluating signalized intersections. The Highway Capacity Manual (Transportation Research Board, 2000) strongly recommends that any analysis of signalized intersections include both a capacity analysis and an LOS analysis in order to obtain a complete picture of existing intersection operations. In other words, the CMP analysis should be viewed as a cursory assessment of signalized intersections; further data would need to be collected in order to determine the severity of problems for a specific traffic signal operation.