We Can Fix That with Data

Speed limits are perhaps are worse situation than many of you realize. There is very little engineering that goes into a regulatory (black on white) speed limit. While designers use a “design speed” that speed is only used to determine the minimum conditions for the worst situation. E.G. the tightest curve possible at the design speed. According the the Manual on Uniform Traffic Control Devices (MUTCD) the official way to set a speed limit is described as “When a speed limit is to be posted, it should be within 10 km/h or 5 mph of the 85th-percentile speed of free-flowing traffic.”

A speed study determines the 85th-percentile speed. This is done by sitting on the side of the road and measuring how fast motorists are moving when no other motorist is limiting their speed (i.e. not held up by other traffic). The result is the highest speed at which 85 percent of the traffic is moving rounded to the nearest 5 mph increment. (Some agencies may also always round down or up). So in other words, you the driver set the speed limit. The only engineering reason the speed is set lower is to allow for road conditions (steep grade, deadman’s curve), roadside development (shopping district, residential), or similar as listed in the MUTCD.

Fascinating — it’s the old “build it and watch how they use it” approach. From the interesting comments in this interesting post on why you should just wear your freaking seatbelt already (via Metafilter).