Highway travel times increased between 1991 and 2001 in 72 of the 75 urban areas studied by the Texas Transportation Institute. The average Travel Time Index (TTI) for these areas in 2001 was 1.39, an increase from 1.29 in 1991 . This means that in 2001 it took 39 percent longer, on average, to make a peak period trip in urban areas compared with the time it would take if traffic flowed freely (box 3-A).
Travel times tend to deteriorate as urban area size increases (figure 3-1). For instance, Los Angeles, California, had the highest TTI (1.83) in 2001, while Anchorage, Alaska, and Corpus Christi, Texas, had the lowest (each 1.05). Of the 30 urban areas with the highest index in 2001, only three had a population under 1 million: Austin, Texas (1.31); and Tacoma, Washington, and Charlotte, North Carolina (1.27 each). At the other end of the spectrum, urban areas of over 1 million people with low indexes include Buffalo-Niagara Falls, New York (1.08) and Oklahoma City, Oklahoma, and Pittsburgh, Pennsylvania (1.10 each).
Between 1991 and 2001, the greatest increases in TTI generally occurred in large urban areas, while the increases were more moderate in the very large, medium, and small areas1 (figure 3-2). Overall, the average index for large urban areas increased by 11.9 percent, while that for medium urban areas was up by 8.2 percent. In small and very large areas, the increases were 4.7 percent and 7.0 percent, respectively.
In urban areas, where highway infrastructure is typically well developed, the principal factor affecting travel times is highway congestion resulting from recurring and nonrecurring events. Recurring delay is largely a phenomenon of the morning and evening commutes, although in some places congestion may occur all day and on weekends. National estimates, based on model simulations, of the effect of nonrecurring events on freeways and principal arterials suggest that about 38 percent are due to crashes, followed by weather (27 percent), freeway work zones (24 percent), and breakdowns (11 percent) .
1. Chin, S.M., O. Franzese, D.L. Greene, H.L. Hwang, and R. Gibson, “Temporary Losses of Highway Capacity and Impacts on Performance,” Oak Ridge National Laboratory, 2002.
2. Texas A&M University, Texas Transportation Institute, 2003 Urban Mobility Report (College Station, TX: 2003).
1 Very large urban areas have a population over 3 million; large urban areas, 1 million to 3 million population; medium urban areas, 500,000 to 1 million; and small urban areas, less than 500,000.