Some states that have raised the speed limit on rural interstate highways from 55 mph to 65 mph, following enactment of the Surface Transportation and Uniform Relocation Assistance Act in 1987, have restricted truck speeds by imposing Differential Speed Limits (DSL), in which the maximum speed limit for trucks is 55 mph and that for passenger cars is 65 mph. The purpose of this study was to assess the nature and extent of the effects of DSL on vehicle speeds and accident characteristics. Before and after data at test and control sites operating under DSL and non-DSL conditions were statistically analysed to determine whether speed and accident characteristics changed significantly as a result of the higher speed limit with DSL. Briefly, some of the findings were as follows: In states where DSL was imposed, there was no significant increase in the mean speeds of trucks. The DSL caused an increase in the average speeds of passenger cars of about 1 to 4 mph. Speed fluctuations within the non-truck traffic stream decreased. Speed variances for all vehicles were higher on Virginia highways with DSL (65/55 mph) when compared with those for similar highways in West Virginia operating under 65/65 mph. No significant reduction in the rate of non-truck/truck accidents, or any two-vehicle accidents, occurred with DSL, compared with those on highways operating with 65/65 mph. There was no evidence indicating that the increase of the maximum speed limit to 65 mph for passenger cars on the rural interstate systems in the states studied has directly resulted in a significant increase in fatal, injury and overall accident rates. Rear-end accidents were relatively higher in Virginia than in West Virginia, suggesting that the DSL caused more rear-end accidents especially between cars and trucks. The rate of two-vehicle accidents reduced by a larger amount in West Virginia after the implementation of the 65/65 mph strategy, than in Virginia after the implementation of the 65/55 mph strategy. (A)
Abstract