To keep up with just one year of increased traffic demand, the average city would have to add 37 more lane miles of highway, according to a study of 68 urban areas released Tuesday by the Texas Transportation Institute.
The 1999 Annual Mobility Report measured urban mobility in several different ways. It shows conditions from 1997, the most recent year for which information is available.
One measure, the Travel Rate Index, shows the difference between a trip taken during peak travel times and the same trip made in uncongested conditions. This peak-period time penalty more than doubled between 1982 and 1997 in the cities studied. It increased even more in the largest cities.
"Mobility levels are declining just about everywhere, but they're declining fastest in those cities where transportation investment fails to keep pace with population growth," said study author Tim Lomax. "As we see it, a healthy economy means more travel by people and freight. If transportation systems aren't expanded, this travel takes longer and is less reliable."

The cities with the worst Travel Rate Index were Los Angeles, Seattle-Everett, San Francisco-Oakland, Washington, DC, and Chicago.
When researchers looked at the cities with the highest increase in the Travel Rate Index in the last five years, Laredo, TX, topped the list, with a 150% increase. Laredo has experienced significant traffic growth, particularly in truck traffic, because of increased trade between the United States and Mexico. Other cities that have experienced rapid worsening of mobility in recent years include Atlanta, Cleveland and Kansas City.
Over the long term, 1982 to 1997, Indianapolis experienced the biggest increase, up 1,000%.
Researchers note that there are no simple solutions. The study did find that overall mobility in urban areas is better if areas try to build new roads at a pace similar to that of the growth in traffic. There is a correlation between the lane-mile addition "deficit" and mobility levels.
Other tactics the report addresses include lowering the number of vehicles on the road, for instance through mass transit; changing the time that vehicles use the road, such as using congestion pricing on toll roads; and getting more vehicles past a spot on the road, using intelligent transportation systems.