Congestion References

Congestion Data

These data were collected and calculated by the Texas Transportation Institute as a part of its annual urban mobility report. Most of the raw data are from the US DOT's Highway Statistics annual reports, available on line. The data are not entirely reliable:

Urban area populations and land areas do not always tally with the carefully-measured numbers published by the U.S. Census Bureau;

States estimates of vehicle miles of travel on freeways and other state highways are fairly accurate, but estimates of local miles are crude and may vary wildly from year to year. These affect total vehicle miles of travel, so major changes from one year to another should be ignored.

The calculated numbers, including travel times, delays, and other measures of congestion, are questionable for several reasons. The calculations assume, for example, that all freeway lane miles have the same capacity, when in fact newer freeways have much higher capacities than older ones due to wider lanes, gentler curves, and a profusion of auxiliary lanes for on- and off-ramps. This means that the calculations for cities with older freeways, such as New York, may underestimate actual congestion while calculations for cities with newer freeways, such as Houston and Phoenix, may overestimate congestion.

The Institute also changed its calculation methods recently but has not retroactively applied the new methods to years before 2000. Attempts to compare changes in congestion over time should be wary of this change; Portland, for example, had the second-fastest growth in congestion before 2000, but much more modest growth after 2000.

Finally, the annual reports were once financed primarily by federal and state departments of transportation. In the past several years, however, the American Public Transportation Association (the chief lobbying group for the transit industry) has provided funding for the report. This has led the Institute to overstate the in reducing congestion. The Heritage Foundation published a detailed critique of this portion of an earlier version of Institute's analysis written by Wendell Cox and Randal O'Toole.

The following Texas Transportation Institute files are in PDF format unless otherwise noted:

The Guide to the American Dream

Congestion Data