Traffic Congestion

Measures of Traffic Congestion

The FHWA officially measures traffic congestion in terms of a travel time index and delay. The travel time index consists of the ratio of the time it takes to travel during peak infrastructure use to the time it takes to travel the same distance when traffic flows freely. For example, if a commuter must spend 35 minutes traveling from point A to point B during a holiday, but only spends 20 minutes traveling that same distance on a regular weekday, that travel time is calculated by dividing 20 by 35, and travel time is 1.75 in the travel time index. This measure accounts for both reoccur-ring (i.e., late afternoon commuter traffic) and incidental (i.e., traffic accidents) travel interruptions. The Texas Transportation Institute found that the Los Angeles-Long Beach-Santa Ana, California, urban area had a travel time index of 1.84, while the Corpus Christi, Texas, urban area had a travel time index of 1.04, indicating that the former urban area experienced more congestion than the latter.

The delay measure consists of annual hours of delay (hours of extra travel time due to congestion), annual delay per capita (hours of extra travel time divided by area population), and annual delay per road user (extra travel time hours divided by peak period road users). For example, the Texas Transportation Institute's regular analysis of traffic congestion in the largest 85 urban areas demonstrated that American travelers lost 47 hours in annual hours of delay, up from 16 annual hours of delay in the 1982 traffic congestion survey.

In addition to the time travel index and delay, recently the FHWA began measuring travel time reliability, a measure of variation in travel time taking into account unpredictable congestion (i.e., a traffic light malfunctions, leaving cars backed up at an intersection until police arrive to direct traffic). Travel time reliability varies over time because conditions such as the weather and work zones vary over time. Traveling in an area with highly variable travel time causes travelers to leave extra time, or buffer, in their schedule for possible delays even though at worst this may result in a traveler arriving early instead of on time, this is still time the traveler might have used more productively elsewhere.

...