Safety Impacts and Other Implications of Raised Speed Limits on High-Speed Roads

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This report describes the analyses performed and results obtained by a study of safety and other impacts of speed limit changes on high-speed roads.
Safety-related analyses were based on a comprehensive framework of the disaggregate relationships between speed limits, driver speed choices, crash occurrence and crash severity. Using a variety of datasets, the project conducted numerous statistical analyses to elucidate and quantify these relationships. It was found that a speed limit increase on a high-speed road is generally associated with a less-than-equivalent increase in average vehicle speed: a 10 mi/h speed limit increase, for example, corresponds to average speeds around 3 mi/h higher. The project identified a relatively small but statistically significant correspondence between speed limits and total crash rates: a speed limit increase from 55 to 65 mi/h on an “average” high-speed road section would be associated with a crash rate increase of around 3%. Finally, the project found a statistically significant association between speed limits and the distribution of injury severities following a crash. For example, the project’s models predict that a speed limit increase from 55 to 65 mi/h on the average section would be associated with a 24% increase in the probability of an occupant being fatally injured, once a crash has occurred. Considering that the crash rate itself increases slightly with a speed limit increase, overall fatality rates are predicted to rise by slightly higher percentages. However, the association between speed limit and injury severity dominates the overall fatality rate result.

The magnitude of some of these relations between speed limit changes and safety factors is subject to uncertainty because of data limitations. In particular, most of the available datasets had data on roads with different speed limits, rather than before-after data on roads that experienced speed limit changes. However, observing injury rate changes on a single road after a speed limit increase is not the same as observing injury rate differences across two existing roadways with different speed limits. Average speed differences are estimated to be higher in the latter, on the order of 6 mi/h (rather than 3 mi/h) for every 10 mi/h difference in speed limit. This data distinction is expected to translate into over-estimates of the estimated magnitude of the injury severity distribution changes following a speed limit change. Nonetheless, even after making allowances for such effects, the relationship between typical speed limit changes on high-speed roads and the injury severity distribution would in many cases remain statistically and practically significant.

The investigation of non-safety impacts relied on published literature, unpublished reports by state DOTs of speed limit change impacts, and results of surveys of state DOT and police officials. The study considered economic, environmental and other non-safety impacts of speed limit changes. The higher speeds resulting from a speed limit increase lead to travel time savings that have an economic value. The vehicles most likely to experience such savings are those making long distance trips primarily in rural areas, where vehicle speeds are not significantly constrained by congestion. On the other hand, vehicles have higher operating costs at higher speeds; for a typical passenger car trip, the operating cost increase associated with a speed limit increase of 55 to 65 mi/h is roughly half the value of the reduced travel time. Approaches for determining the economic costs of injuries and fatalities were also reviewed. Little is known regarding the air quality and noise impacts of speed limit changes; the few available studies suggest that these impacts are very small to negligible. No reliable information was found regarding possible impacts of speed limit changes on business and commerce. Similarly, available data do not allow definite conclusions to be drawn regarding the impacts of differential light/heavy vehicle speed limits.