Field Test of the Impact of Setting and Enforcing Rational Speed Limits: Final Report for Gulfport, Mississippi–Demonstration Community
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Field Test of the Impact of Setting and Enforcing Rational Speed Limits: Final Report for Gulfport, Mississippi–Demonstration Community

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  • English

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    • Alternative Title:
      Field test of the impact of setting and enforcing rational speed limits in Gulfport, Mississippi
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    • Edition:
      Final report
    • Abstract:
      To address concerns regarding the need to restore credibility to speed limits, the National Highway Traffic Safety Administration (NHTSA) and the Federal Highway Administration (FHWA) jointly undertook tests of “rational speed limits,” which have been established by a formal engineering review that starts with the 85th percentile speed of free flowing traffic, but could be set as low as the 50th percentile, depending on roadway geometry, land use, access, etc. The objective of these tests was to determine whether speed limits so set, when combined with well-publicized and targeted enforcement, result in greater compliance, more uniform speeds, and improved safety. This report presents results of the demonstration and evaluation of rational speed limits on a 7.5 mile segment of U.S. Route 49 in Gulfport, Mississippi. Following data collection and engineering analyses, the speed limits on various portions of the demonstration road in Gulfport were variously raised from 5 to 15 mph. The increases in limits were accompanied by public information and education and a stricter enforcement of the raised limits. Data on speeds, crashes, citations and enforcement hours were collected in both Gulfport and a comparison community, prior to, and quarterly during the one-year demonstration period. The principal findings from the Gulfport, Mississippi, demonstration follow: (1) Although a small proportion of drivers continued to violate the rational limits by more than 10 mph after the rational limits were implemented, the number of such speed violations was reduced by three quarters. Thus, rational limits resulted in better compliance with the law. (2) The small increases observed in both mean and 85th percentile speeds in the demonstration community, but not in the comparison community, suggest that implementing rational speed limits may not lead to increases all the way up to the newer raised limits, but that some increases in speeds are to be anticipated. (3) The small changes observed in the standard deviation of speeds in the demonstration community (increases of 1-3 mph in 4 locations) and similar changes in the coefficient of variation suggests that raising the speed limit, even with strict enforcement, may not result in decreased speed variation. (4) A reduction in the proportion of extreme speeders (95th percentile speeders) was observed only on the road segment where the limit was increased the most (+15 mph). The reasons the proportion of extreme speeders did not decline on the roadways where limits were increased by smaller amounts are not clear. (5) The average monthly frequency of crashes in the demonstration community was lower during the demonstration year when compared to crashes in just the year immediately preceding the demonstration. However, the average monthly frequency of crashes was higher in the demonstration year than it was in the three year period preceding the demonstration year. Similar changes were observed in the comparison community where the speed limits were not changed, but the availability of just 2 years of baseline data there limited assessment of the demonstration site effect. Further studies (ongoing in 6 other communities) are required to determine the full impact of rational speed limits. /Abstract from report summary page/
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