Several performance measures are used to measure and track congestion, but they all relate to what travelers find important—travel time and the day-to-day variation in travel time. The Texas Transportation Institute (TTI) is a leader in developing measurements for determining congestion.  Congestion Measures can be subdivided into Mobility Measures and Reliability Measures.

Mobility Measures

These are the five most common measures for mobility:  

Volume-to-Capacity Ratio (V/C Ratio): the volume divided by capacity.  For the Level of Service (LOS) calculations, volume is often taken to be the 30th yearly highest. 

The Level of Service (LOS): a grade from A to F – with A being free flow and F being very congested – that indicates how well the roadway or intersection is serving its intended traffic.  LOS is based on a volume to capacity (v/c) ratio and has long been used as the primary measure of congestion for planning purposes.  See the Highway Capacity Manual for more details.

Travel Time Index: ratio of average peak travel time to an off-peak (free-flow) standard, in this case 60 mph for freeways. For example, a value of 1.20 means that average peak travel times are 20% longer than off-peak travel times.

Travel Delay: the amount of extra time spent traveling due to congestion.

Percent of Congested Travel: the congested vehicle-miles of travel divided by total vehicle-miles of travel. A relative measure of the amount of travel affected by congestion.