Highway Congestion and Crash Scans
As part of CMAP's Congestion Management Process, select performance measures for the regional freeways have been created from five minute speed data obtained from the Illinois Department of Transportation (IDOT) and Traffic.com. These performance measures include congestion scans, travel time index, planning time index, and congested hours.
The Chicago region traffic congestion scans were created by averaging five minute speed data obtained from freeway loop detectors, radar detectors, and mobile technology data. The traffic scans display average speed on a freeway corridor as a function of time of day (the x-axis) and location (the y-axis) along a freeway corridor.
Crash scans show the rate of total highway crashes per 100 million vehicle-miles traveled (VMT) for 1/10 mile segments. The purpose of these crash scans is to show highway planners and system managers how crashes are related to congestion. Understanding where crashes are concentrated can help planners and engineers focus on improving those locations with the worst performance. Solutions may involve safety improvements, but are just as likely to involve traffic operations or highway geometry improvements.
The travel time index is a measure of average congestion and indicates the average extra travel time required during peak period congestion. Travel time index is a ratio of the average peak-period travel time compared to the free-flow travel time. For example, a value of 1.20 means that average peak travel times are 20 percent longer than free-flow travel times.
The planning time index is a measure of reliability and is defined as the ratio of the total time needed to ensure a 95 percent on-time arrival to the free-flow travel time. For example, a value of 1.4 means that, to arrive on-time 95 percent of the time, a traveler should budget an additional eight minutes for a trip that takes 20 minutes during off-peak, free-flow conditions.
Congested hours are an indication of how many hours per day a facility is congested. Technically, "congested hours" are defined as the average number of hours in which at least 20 percent of the vehicle miles traveled (VMT) on the instrumented segment is congested. For this measure, congestion is defined to occur when link speeds are less than 50 mph.
Data provided by IDOT, Illinois Tollway, and HERE. CMAP's Todd Schmidt created the performance measures.