The changes in the before and after percentile speeds for each of the 4 sites where speed limits were raised to 65 mi/h (105 km/h) is shown in figure 36. Examination of figure 36 reveals that driver speeds at these sites increased between 1 and 3 mi/h (1.6 and 4.8 km/h). Although not shown, the changes in the percentile speeds at the comparison sites, where the 55 mi/h (89 km/h) speed limit remained in effect, were generally 1 mi/h (1.6 km/h) or less.
Figure 36 also reveals that the 99th percentile speed, which contains the highest speed drivers, either did not change or was 1 to 2 mi/h (1.6 to 3.2 km/h) lower after the speed limit was raised to 65 mi/h (105 km/h) at the experimental sites.
Shown in figure 37 are the changes in the mean speed and the standard deviation of speeds at each rural Interstate site. While there was an increase in the mean speed, there was a reduction in the standard deviation of speeds at three of the sites.
As expected, increasing the speed limit reduced the number and percentage of drivers exceeding the posted speed limit. The average reduction in drivers exceeding the speed limit was 39 percent. In addition, there was a 41 percent reduction in drivers exceeding the speed limit by 10 mi/h (16 km/h), and a 4 percent reduction in drivers exceeding the speed limit by 20 mi/h (32 km/h). While these are average values, the findings are similar at each of the four sites.
Repeated speed measurements were conducted before the speed limits were changed, and at 3-month intervals thereafter at several rural Interstate sites. Shown in figure 38 are the 85th percentile speeds for 2 experimental and 1 comparison site. The data indicate that the 85th percentile speeds increased from 1 to 3 mi/h (1.6 to 4.8 km/h).
To provide an indication if raising the speed limit on rural Interstate increased vehicle speeds on a contiguous urban Interstate, before and after data were collected on a section of rural Interstate where the speed limit was raised, and on an urban Interstate and rural limited access highway where the posted speed limit remained at 55 mi/h (89 km/h). Before and after data showing the effects on the 85th percentile speeds at these sites is given in figure 39. Although the sample is too small to draw conclusions, it does not appear that the 85th percentile speeds increased on either of the 55-mi/ (89-km/h) sections.
One concern often expressed about raising Interstate speed limits is the pullover effect which suggests that speeds may increase on nearby rural two-lane highways. To examine this effect, a rural 2-lane site within 1 mi (1.6 km) of an interchange was selected for study. In addition, data were collected 3 mi (4.8 km) from another rural interchange. The mean speeds for the Interstate sections, as well as the 2-lane sections, are presented in figure 40. While the mean speed on the rural Interstate increased by 2 mi/h 13.2 km/h) after the 65-mi/h (105-km/h) limit was posted, there was little change in the mean at the site within 1 mi (1.6 km) of the interchange. The mean speed at the site located 3 mi (4.8 km) from the interchange fluctuated overtime. At the end of the 1-yr after period, the mean speed at this site wash mi/h (3.2 km/h) higher than it was prior to the speed limit change on the Interstate section.
Overall, there appears to be a 1- to 2-mi/h (1.6- to 3.2-km/h) increase in the mean and most percentile speeds on rural Interstate sections after the speed limits were raised. Based on the data collected in this study, there does not appear to be an increase in the 99th percentile speed, which contains the highest speed drivers.
Before and after accident data were also collected for the Interstate experimental and comparison locations. At each site, reported accidents were obtained for a 3-yr before period and a 1-yr after period. The G test revealed that the accident history at the comparison sites was compatible with the accident changes at the experimental sites for three of the four locations. The paired comparison ratio method was used to estimate the safety effects at the three comparable locations. The before and after method was used at the fourth location. A summary of the accident results is shown in table 13.
While accidents decreased at one site, there was an apparent increase in total accidents at 3 sites, ranging from 30 to 48 percent. While the number of sites is too small to draw inferences, it appears in this limited data set that accidents at the study sites increased overall by 25 percent. Injury accidents appeared to increase by 12 percent; however, the sample was too small to draw conclusions. No general statements should be made from this analysis as only one year of after data were available.
Table 13. Summary of Interstate accident results. -------------------------------------------------------------------- -----Number of Accidents----- Experimental Comparison 3-yr 1-yr 3-yr 1-yr Percent Z Site Before After Before After Change Value p Value -------------------------------------------------------------------- 1 229 92 573 235 -2.0 -0.14 0.89 2 438 219 1,283 432 48.5 3.79 >0.001 3 157 86 523 211 35.8 1.95 0.05 4 391 169 - - 29.7 2.82 0.007 --------------------------------------------------------------------
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