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Effects of raising and lowering speed limits on selected roadway sections
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    Final report
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    The objective of this research was to examine the effects of raising and lowering posted speed limits on driver behavior for urban and rural nonlimited access highways. Sites selected for study were furnished by the participating States. The study was conducted during the period from October 1985 to September 1992, when the maximum speed limit was 55 mi/h (89 km/h) on nonlimited access highways. During this period, the States and localities lowered and raised posted speed limits on short roadway segments, typically less than 2 mi (3.2 km) in length. The general types of sites included in the study were short sections, i.e., 0.5mi (0.8-km) segments in rural communities, I-mi (1.6-km) sections in urban and rural communities, and 2-to 12-mi (3-to-19-km) rural sections where speed limits were raised. The study included the collection of driver behavior and crash data in 22 States. The data were collected at 100 sites on nonlimited access highways, consisting of 172 mi (277 km) where speed limits were either lowered or raised, and at 83 comparison sites, consisting of 132 mi (213 km) where no changes in the posted speed limits were made. Changes in the posted speed limits ranged from lowering the speed limit by 5, IO, 15, or 20 mi/h (8, 16, 24, or 32 km/h) to raising the speed limit by 5, 10, or 15 mi/h (8, 16, or 24 km/h). Only one change in the posted speed limit was made at each site during the study. There is statistically sufficient evidence in this dataset to reject the hypothesis that driver speeds do not change when posted speed limits are either raised or lowered. However, the differences in speeds, less than 1.5 mi/h (2.4 km/h), are not sufficiently large to be of practical significance, and are due primarily to large sample sizes. Although the changes in vehicle speeds were small, driver violations of the speed limits increased when posted speed limits were lowered. Conversely, violations decreased when speed limits were raised. This does not reflect a change in driver behavior, but a change in how compliance is measured, i.e., From the posted speed limit. There is not sufficient evidence in this dataset to reject the hypothesis that crash experience changed when posted speed limits were either lowered or raised.
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