Traffic engineers believe that the 85th percentile speed is the ideal speed limit because it leads to the least variability between driving speeds and therefore safer roads. When the speed limit is correctly set at the 85th percentile speed, the minority of drivers that do conscientiously follow speed limits are no longer driving much slower than the speed of traffic. The choice of the 85th percentile speed is a data-driven conclusion — as noted Lt. Megge and speed limit resources like the Michigan State Police’s guide — that has been established by the consistent findings of years of traffic studies.
Yet most speed limits are set below the 85th percentile speed.
Last Sunday, I decided to see what kind of gas mileage I could get on my motorcycle. I drove 90 minutes into downtown Vancouver, keeping to the speed limit the entire time (except for the 30 seconds I didn’t and this happened). I got passed by everybody. Semis, minvans, old men in Buicks. Even thought the speed limit was 100kph (about 60mph), literally no one was doing the speed limit.