Impact of Speed Limit Change on Driving Speed and Road Safety on Interurban Roads: Meta-Analysis

A meta-analysis was conducted to study the effect of a change in the speed limit on mean driving speed and safety on interurban roads. Information from 108 research results—documented in a database collected by Elvik et al. in 2004—was analyzed. The link between accident ratio (after–before) and the mean driving speed ratio was studied by estimating the power parameter in the Nilsson power model. A modified power model was applied to estimate the link between speed limit (rather than mean driving speed) ratio and accidents ratio. When the speed limit was raised, the mean driving speed ratio (mean driving speed after to mean driving speed before) was on average 1.04 (SD = 0.04), significantly higher than 1 but smaller than the speed limit ratio (average = 1.17; SD = 0.05). The power parameters in Nilsson’s power model and in its modified version were 3.73 (SD = 2.61) and 0.99 (SD = 0.46), respectively. When speed limit was reduced, the mean driving speed ratio was 0.93 (SD = 0.04), significantly smaller than 1 but larger than the speed limit ratio (average = 0.82; SD = 0.04). The corresponding power parameters were 2.81 (SD = 0.65) and 1.24 (SD = 0.44). This paper summarizes research on the link between changes in speed limit, driving speed, and safety. It was found that the average speed changed in the same direction as the speed limit, but the magnitude of the change was smaller. As a result, the power parameters in the second version of the power model were lower than those in the original Nilsson’s model, and so the average change in safety is nearly proportional to the change in speed limit.