It's also the way civil engineers recommend speed limits be set. I think the current number they use is 85th percentile.

> The 85th percentile speed is the single most influential indicator of what is safe and reasonable, and it is used to determine the speed limit by rounding to the nearest 5 mph increment, and posting the speed limit at that speed. If there are sufficient conditions not readily apparent to the driver, as determined by a registered engineer, the posted speed limit can be lowered by 5 mph, and such a decision must be clearly documented in the E&TS.

https://dot.ca.gov/programs/safety-programs/setting-speed-li...