Speed limits are something you might just take for granted. I know I did, until I taught my kids how to drive. We gradually went from low-speed city roads to the countryside, the bypass, and then the interstate, while I pointed out why conditions are different for each. The posted speed limit is affected by how curvy, crowded, and wide the road is, how many turnoffs and intersections there are, visibility, and a few other factors. The procedure for setting speed limits also includes engineers studying the "prevailing speed."
Our hypothetical engineer figures out how many drivers are on the road and how fast they drive. Once that data’s collected, it’s plotted out to help determine how fast the majority of drivers are traveling. In particular, the engineer wants to determine the “85th percentile speed”—meaning how fast 85 percent of the cars travel.
The 85th percentile speed, a blogger at engineer company SEH put it, is called the “prevailing speed,” because it’s considered the safest speed to travel. Interesting, right?
There are other factors that go into setting speed limits. And some places throw all the data out the window anyway, like speed traps where the limit is ridiculously low and long highways out west where the limit is pretty high. Read about how speed limit decisions are made at Jalopnik.