Changes in motor vehicle occupant fatalities after repeal of the national maximum speed limit

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Abstract

Trends in motor vehicle occupant deaths over 8 years were studied for 24 states that raised interstate speed limits and seven states that did not following the 1995 repeal of the US National Maximum Speed Limit. Fatalities on interstates increased 15% in the 24 states that raised speed limits. After accounting for changes in vehicle miles of travel, fatality rates were 17% higher following the speed limit increases. Similar increases were reported following the 1987 speed limit increases on rural interstates. Deaths on roads other than interstates were essentially unchanged.

Introduction

The regulation of speed limits in the United States historically has been the responsibility of individual states. During the early 1970s, many states had posted speed limits on rural roads as high as 75 mph. In 1974, however, in response to a national energy shortage, the US Congress established a National Maximum Speed Limit (NMSL) of 55 mph. States were required to monitor travel speeds on major highways to assess motorists’ compliance with the NMSL. Over time, the energy shortage eased, but the NMSL was preserved in large part because of its life-saving benefits (Transportation Research Board, 1984). Despite the safety benefits, there was political pressure to repeal the NMSL, and in 1987 Congress passed legislation allowing states to increase speed limits to 65 mph on rural interstates. Finally in December 1995, the NMSL was repealed, returning to the states full authority to set speed limits on all roads.

A recent report issued by the National Research Council reviewed current practices for setting and enforcing speed limits (Transportation Research Board, 1998). The authors concluded that ‘decisions about appropriate speed limits depend on judgments about society’s tolerance for risk, valuation of time, and willingness to police itself.’ They cautioned, however, that the effects of these decisions on driving speeds and safety should be closely monitored.

When the NMSL was introduced in 1974, motor vehicle fatalities declined dramatically. According to the National Highway Traffic Safety Administration (NHTSA), there were 45 196 motor vehicle fatalities in 1974 compared with 54 052 in 1973 (Partyka, 1991). Vehicle miles of travel also declined by approximately 3%, but the lower speeds mandated by the NMSL were credited with most of the decline in fatalities (Department of Transportation, 1980, Transportation Research Board, 1984). After Congress relaxed the NMSL in 1987, 40 states raised speed limits on rural interstates to 65 mph, and fatalities on these roads increased. A number of studies, employing different methods, were undertaken to evaluate the 1987–88 speed limit increases, with estimates of an increase in fatalities ranging from 15 to 30% (McKnight et al., 1989, Garber and Graham, 1990, Baum et al., 1991, NHTSA, 1992).

After Congress repealed the NMSL on December 8, 1995, a number of states moved quickly to raise speed limits not only on rural interstates but also on urban interstates and other roads previously posted at 55 mph. Speed limits of 70 or 75 mph became common on rural interstates. Speed limits of 65 mph became common on urban interstates. Several preliminary evaluations of these changes have been conducted, but they have been limited to at most 1 year of experience under the higher speed limits.

Farmer et al. (1997) reported a 12% increase in motor vehicle occupant fatalities and a 17% increase in fatality rates (per mile driven) on interstates in 12 states that raised interstate speed limits within 4 months of the NMSL repeal. Trends in fatalities over 7 years were compared for these 12 states and a comparison group of 18 states that did not raise speed limits. Increases in fatalities on roads other than interstates were small and not statistically significant, but the overall effect of the NMSL repeal on all roads in the 12 study states was a statistically significant 6% increase in fatalities. This study, however, did have some limitations. For some of the study states, the analysis was limited to only 9 months after the speed limit increases, and there were insufficient data to analyze effects in individual states. Also, there were clear geographic differences between the study and comparison states. All but one of the study states were west of the Mississippi River, whereas most of the comparison states were east.

In February 1998, NHTSA (1998a) issued a report to Congress on the effects of the NMSL repeal. The study concentrated on 32 states that raised speed limits in 1996, including some that raised speed limits only on selected roadway segments. The conclusion was that ‘states that increased speed limits in 1996 experienced approximately 350 more Interstate fatalities than would have been expected based on historical trends—about 9% above expectations.’ This estimate, although similar to that of Farmer et al. (1997), is open to criticism on at least three points. The estimate is based on fatalities that occurred anytime during 1996, even though some states did not begin raising speed limits until August. Although the fatality trends for a comparison group of states that did not raise speed limits were noted, these trends were not accounted for in the calculation of the above estimate. Finally, there was no attempt to adjust the estimate for changes in vehicle miles traveled.

High travel speed, rather than the posted speed limit, is the factor that contributes to both the likelihood and severity of a motor vehicle crash (Bowie and Walz, 1994, Moore et al., 1995, NHTSA, 1998b). Therefore, it is important to know whether drivers actually respond to higher speed limits by driving faster. Studies conducted after the 1987 speed limit increases on rural interstates reported jumps in mean travel speeds of 2 or 3 mph, even though speed limits were raised by 10 mph (Freedman and Esterlitz, 1990, Freedman and Williams, 1992). However, a seemingly small increase in the mean speed can signal a major increase in the proportion of vehicles traveling at high speeds. For example, Freedman and Esterlitz (1990) observed only 8% of cars exceeding 70 mph just prior to the speed limit increase in Virginia but observed 25% 1 year later.

Retting and Greene (1997) sampled speeds on urban freeways in Riverside, California and Houston, Texas just prior to the speed limit increases of December 1995 and again 3 months later. The mean and 85th percentile speeds of cars on these roads increased during the 3 months by only 2 or 3 mph, but the percentage of cars exceeding 70 mph increased from 29 to 40% in Riverside and from 15 to 33% in Houston. At the end of 1996, speeds were measured again at these same locations. The percentage of cars exceeding 70 mph had leveled off at 41% in Riverside but increased further to 50% in Houston. NHTSA’s (1998a) report on the NMSL repeal summarized individual studies conducted by 10 states, in addition to the nationwide study of fatality trends. Each of the ten states reported increased speeds, 2 or 3 mph on average, on roads where speed limits had been raised. It seems, then, that increases in travel speed following the 1996 speed limit increases were similar to those observed for 1987–88.

The present study is an expansion of the earlier studies of the NMSL repeal. It assesses the effect of 1996 speed limit increases on motor vehicle occupant fatalities through 1997, 2 years after the repeal. Trends over time in fatalities and fatality rates per mile of travel are compared for states that raised speed limits and states that did not, but the methodology takes into account the timing of speed limit increases in each state. States move from the comparison group to the study group as the time series analysis progresses, thus reducing geographic differences in the two groups. Also, to address the hypothesis (Lave and Elias, 1994) that increases in fatalities on roads with raised speed limits are offset by reductions in fatalities on roads where speed limits were not raised (due to increased enforcement or shifts in traffic), this study examines trends on both interstates and roads other than interstates.

Section snippets

Method

Data on motor vehicle occupant deaths in each state for each month beginning January 1990 and ending December 1997 were extracted from the Fatality Analysis Reporting System (FARS), a national census of fatal crashes on public roads. Data on vehicle miles of travel for the same time period were provided as monthly estimates by the Federal Highway Administration. Monthly estimates of the number of employed people in the US civilian population were obtained from the Bureau of Labor Statistics.

Results

Quarterly motor vehicle occupant fatality counts on interstates and freeways are plotted in Fig. 1 for the four groups of states that raised speed limits in 1996. The seasonal pattern is evident in each group, as is a general upward trend beginning in 1994. However, the increases from 1995 to 1996 were greater than would have been predicted by the trend. For the group of states that raised speed limits in December 1995, there was a noticeable jump in first quarter deaths, from 343 in 1995 to

Discussion

Increasing the speed of a motor vehicle reduces the time the driver has to respond to an emergency, increases stopping distance, and increases the likelihood of crashing as well as crash severity (Solomon, 1964, Haddon and Baker, 1981, Joksch, 1993, Moore et al., 1995, Ranta and Kallberg, 1996).

Travel speeds on interstates increased after the 1995–96 speed limit increases (Retting and Greene, 1997, NHTSA, 1998a). Past research has associated similar increases in travel speeds with increases in

Acknowledgements

This work was supported by the Insurance Institute for Highway Safety.

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