| Peak-hour | Peak-hour | Average | |
| travel time | miles traveled | daily | |
| under congested | under congested | vehicles | |
| Year | conditions | conditions | per lane |
| percent with V/SF greater than or equal to 0.80 | thousands | ||
| 1975 | 41 | 23 | na |
| 1978 | 48 | 29 | na |
| 1980 | 52 | 28 | na |
| 1982 | 53 | 28 | na |
| 1984 | 55 | 30 | 9.99 |
| 1985 | 61 | 36 | 10.33 |
| 1986 | 63 | 37 | 10.79 |
| 1987 | 64 | 38 | 11.21 |
| 1988 | 67 | 42 | 11.68 |
| 1989 | 69 | 44 | 11.99 |
| 1990 | 69 | 45 | 12.26 |
| 1991 | 70 | 47 | 12.42 |
| 1992 | 70 | 46 | 12.38 |
| 1993 | 53 | 32 | 12.58 |
| 1994 | 68 | 45 | 12.81 |
| 1995 | 55 | 34 | 13.11 |
| 1996 | 54 | 33 | 13.38 |
| 1997 | 55 | 35 | 14.16 |
| 1998 | 56 | 36 | 14.59 |
Source: U.S. Department of Transportation (DOT), Federal Highway Administration (FHWA), Highway Statistics 1998, Chart “Urban Interstate System Congestion Trends,” p. V-40 (DOT, FHWA, Washington, DC, 1999).
Notes: The FHWA uses several procedures to measure congestion on urban interstate highways: the Volume/Service Flow (V/SF) Ratio; Volume per Lane (average daily travel/lane); and hours of delay/1,000 Vehicle Miles of Travel. Data derived from the first two procedures are presented in this table. The V/FS is a computed numerical value based upon traffic volume information and roadway capacity. As this ratio gets larger, traffic slows and eventually stops as the theoretical value of 1.00 (the volume of traffic = service flow capacity of the facility) is approached. A V/SF ratio value of greater than or equal to 0.80 is used to indicate congestion. Methods used to calculate V/SF have been revised based on research that showed that drivers are willing to follow each other more closely and at higher speeds than peviously. Although this change in driving habits occurred over a period of years, the change in procedure occurred abruptly, starting with data for 1993. Thus congestion data for 1993 forward are not strictly comparable to data for previous years. The second measure is calculated from actual counts of average daily travel and the number of lanes per segment of interstate highway. States are required to report annual average daily travel (AADT) for all interstate and principal arterials on a 3-year cycle. AADT is updated annually.
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