As part of the CMAP Congestion Management Process, staff has compiled performance measures for regional freeways. Information is presented both in charts and numerically. To view the charts and data, select a freeway segment from the window pane to the right. Explanations of our performance measure are below and also in the window pane to the right, below the list of freeway segments.
Data Definitions for Highway Performance Measurement Pages
Congestion Scan. The congestion scan shows the time of day and location of weekday congestion on area freeways. The congestion scan shows computed average speed by milepost and time of day for instrumented segments. Values between instrumented points are interpolated.
Travel Time Index: The travel time index indicates the average extra travel time required during peak period congestion. The travel time index is defined as the ratio of the average peak period travel time to a free-flow travel time. The free-flow travel time is the 15th percentile travel time during traditional off-peak times (weekdays from 9am to 4 pm and 7pm to 10pm). For example, a value of 1.2 means that the average peak travel times are 20% longer than free-flow travel times. In this report, the AM peak is 6am to 9am; the PM peak is 4pm to 7pm on non-holiday weekdays. Averages across road sections and time periods are weighted by VMT.
Planning Time Index: The planning time index is a measure of reliability. The planning time index is defined as the ratio of the total time needed to ensure 95% on-time arrival to the free-flow travel time. This measure is computed for the AM peak period (6am to 9am) and the PM peak period (4pm - 7pm) for non-holiday weekdays. For example, a value of 1.4 means that, to arrive on-time 95% of the time, a traveler should budget an additional 8 minutes for a trip that takes 20 minutes during off-peak, free-flow conditions. The planning time index is computed as the 95th percentile travel time divided by the free-flow travel time for each road section and time period. Averages across road sections and time periods are weighted by VMT.
Congested Hours: "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% 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 50mph.
Acknowledgements:
Thanks to the Illinois Department of Transportation, the Illinois Tollway, the Federal Highway Administration, Navteq traffic, and the other firms and institutions involved in the USDOT Mobility Monitoring Program, which made these analyses possible.
Congestion data analysis was conducted by Todd Schmidt of the CMAP staff.