So claiming (as the city of Edmonton loves to do) that “every one-km/h increase in speed leads to a three-per-cent rise in collisions,” is, according to McGee, “simplistic.”
Believe it or not, setting a speed limit too slow for a given location also adds considerable risk.
According to the Institute of Transportation Engineers, the best speed limits are those set at the 85th percentile. That’s the speed up to which 85 per cent of motorists drive even when there are no speed limit signs.
Limits set too far below that cause accidents because some motorists will be puttering along at the posted limit or less, while others are driving at a higher speed that feels more natural. The mix of vehicles at various speeds is risky.
So unless the city knows what the 85th-percentile speed is on the streets in town, and posts its limits accordingly, it could actually be increasing accident risk by keeping limits unnaturally low.