National Capital Region Congestion Report
4th Quarter 2011 Highlights
Understanding the Key Performance Measures
- Delay: Delay is the difference between actual travel time and free flow travel time. It is calculated from all time periods during the reporting quarter, and is also converted to a monetary value using a regional value of time (e.g., $19.24 per hour for 2011).
- Extra Time for On-Time Arrival: This measure reports travel time reliability - the predictability or consistency in travel times as measured from day to day during the AM or PM peak periods. It is calculated as the ratio (expressed as percentage) of the 95th longest travel time of a route during the quarter to the free flow travel time (this ratio is also called Planning Time Index). For example, a 20-minute free flow travel with 200% extra time for on-time arrival indicates that one has to budget a total of 20 * 200% = 40 minutes to arrive on time. Note the changes compared to the same quarter last year are reported in percentage points (pp), not percentage.
- Congested Freeway Lane-Miles: Four congestion levels were determined based on the ratio of actual travel time to free flow travel time (this ratio is also called Travel Time Index): smaller than 1.15 is uncongested, between 1.15 and 1.30 is lightly congested, between 1.30 and 2.00 is moderately congested, and greater than 2.00 is severely congested.