> This is what’s known as “design speed,” and it may be substantially higher than the legal speed limit. Engineers often use the 85th percentile speed (the speed that 85% of drivers are going at or under) as an indication of design speed. According to Staggs’s findings, on Delta Drive, the 85th percentile speed falls at 45 mph, a full ten miles per hour over the posted speed limit.
They came so close to the solution without saying it. The point of the 85% rule is that's what the speed limit is supposed to be. The reason so many people are speeding is that the speed limit is too low, and the right answer is to raise it.
While I don't disagree with the general notion that ideally the "natural" speed of a road should match the intended target speed, especially in the case of existing infrastructure changing the road design isn't always possible easily and/or quickly, and yet there might still be good reasons for lowering the speed on a particular stretch of road. So I still disagree with a blanket "well, then never post a speed limit that's lower than what people would drive at naturally".
They came so close to the solution without saying it. The point of the 85% rule is that's what the speed limit is supposed to be. The reason so many people are speeding is that the speed limit is too low, and the right answer is to raise it.