The study sought to estimate changes in actual driving speed occurring after new speed limits were introduced in Sweden's rural road network.
The effects of speed limit changes were estimated for 7 groups of roads of different types and initial speed limits. To study the effects on the entire road network and not only at specific road sites, a sampling survey was conducted in which speed was measured at randomly selected sites before and after the speed limit changes. Systematic sampling was used to select sites that were widely distributed geographically, though the analysis treats the data as if the sites were selected by simple random sampling. The speed of passing vehicles was generally measured using pneumatic tubes stretched across the road.
The survey results indicate that the mean car speed increased by 3.5 km/h when the speed limit increased by 10 km/h on motorways and 2 + 1 roads. Reducing the speed limit by 10 km/h on 2 + 1 roads and rural roads with a speed limit of 110 km/h resulted in a 2 km/h decrease in mean speed. On rural roads where the speed limit was lowered from 90 to 80 km/h, the mean speed decreased by 3.3 km/h. These changes are statistically significant.
The present results are in line with previous results indicating an average change in mean speed of approximately 2.5 km/h when the speed limit changes by 10 km/h. The confidence intervals were in most instances fairly small, indicating a sufficiently large number of measurement sites.