Speed limit research

Speed limits should be related to the design speed of a road. Low limits on roads built for high speeds are likely to be disregarded, resulting in higher speeds than if a realistic limit is imposed. The Transport Research Laboratory TRL concluded in 1994 that changing the speed limit on motorways to 80mph might not alter traffic speeds by much. It might rise by about 3-4mph provided drivers feel the limit is reasonable. “Although there is no experimental evidence that raised speed limits result in lower speeds ... such effects are said to occur. The explanation ... has to do with the driving public's response to the reasonableness or unreasonableness of the limit. If the limit is raised to something more appropriate to the design speed of the road, then ... some drivers will respond by observing it and speeds fall.”

The TRL found evidence that “drivers driving much faster or much slower than the general traffic stream are more likely to be involved in accidents.” The speed traffic is actually doing is less important than making sure all the vehicles are going about the same speed in relation to one another.

Analysis of the effect of speed on accidents has always been problematical and the TRL’s findings endorses the view that blanket limits can sometimes be unproductive. Limits should reflect the dangers of individual stretches of road and there would be more benefit from reducing the speed of the fastest drivers than reducing speeds for all drivers. This is especially true for urban roads where engineering and enforcement targeting the fastest drivers tends to work.

In a report compiled in 2000 the TRL said: “The scope for reducing accidents by means of speed management depends on the operational characteristics of the road. The often-quoted broad result that a ‘5% reduction in accident frequency results per 1mph reduction in average speed’ has been investigated carefully; although it remains a robust general rule, the percentage reduction in accident frequency per 1mph reduction has been shown to vary according to road type and average traffic speed. It is: about 6% for urban roads with low average speeds; about 4% for medium speed urban roads and lower speed rural main roads; about 3% for the higher speed urban roads and rural main roads. In urban areas the potential for accident reduction is greatest on those roads with low average speeds. These are typically busy main roads in towns with high levels of pedestrian activity, wide variation in speeds, and high accident frequencies.”