Highway travel times increased between 1993 and 2003 in all but 2 of the 85 urban areas studied by the Texas Transportation Institute. The average Travel Time Index (TTI) for the 85 areas in 2003 was 1.37, an increase from 1.28 in 1993 . This means that in 2003 it took 37 percent longer, on average, to make a peak period trip in these urban areas compared with the time it would take if traffic flowed freely (box 5-A).
Travel times tend to deteriorate as urban area population increases (figure 5-1). For instance, Los Angeles, California, had the highest TTI (1.75) in 2003, while Corpus Christi, Texas and Anchorage, Alaska, had the lowest (1.05). Of the 30 urban areas with the highest index in 2003, only five had a population under 1 million: Austin, Texas (1.33); Tucson, Arizona and Charlotte, North Carolina-South Carolina (1.31 each); Bridgeport-Stamford, Connecticut-New York (1.29); and Salt Lake City, Utah (1.28). At the other end of the spectrum, urban areas of over 1 million people with low indexes include: Cleveland, Ohio (1.09); Buffalo, New York, Pittsburgh, Pennsylvania, and Oklahoma City, Oklahoma (all 1.10); and Kansas City, Missouri-Kansas (1.11).
Between 1993 and 2003, the greatest increases in TTI occurred in very large, large, and medium urban areas, while the increases were more moderate in small urban areas1 (figure 5-2). Overall, the average index for very large urban areas increased by 10 points (from 1.38 to 1.48), while the index increased by 9 points in large areas (from 1.19 to 1.28) and by 7 points in medium areas (from 1.11 to 1.18). The TTI in small urban areas increased by 4 points (from 1.06 to 1.10).
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 50 percent are due to crashes, followed by work zones (27 percent), breakdowns (13 percent), and weather (10 percent) .
1. S.M Chin, O. Franzese, D.L. Greene, H.L. Hwang, and R. Gibson, "Temporary Losses of Highway Capacity and Impacts on Performance: Phase 2," Oak Ridge National Laboratory, 2004, table ES-1.
2. Texas A&M University, Texas Transportation Institute, 2005 Urban Mobility Report (College Station, TX: 2005).
1 Very large urban areas have a population over 3 million; large urban areas, 1 million to 3 million; medium urban areas, 500,000 to 1 million; and small urban areas, less than 500,000.