[rootusrootus]

This has been studied to death, seriously, it doesn't need to be argued here. Actual traffic engineers design roads all the time for a particular speed limit. A lot of the actual limits implemented, however, are purely political. And basically ineffective, as has been documented repeatedly. If you want people to go slower, there are easy ways to achieve that through correct road design.

[elipsey]

My understanding is that it is standard practice in traffic engineering to set speed limits to the 85th percentile speed of observed traffic.

http://www.mikeontraffic.com/85th-percentile-speed-explained...

[rootusrootus]

Sure, they design the road with a target (including non-speed-related requirements), then observe, then recommend a speed limit. If it's too fast, they can do things like narrow the lanes, etc, which will slow traffic down. Changing the signs does nothing, more or less, because only about 10% of drivers base their speed on the sign.

Portland is going through this right now. Dropped the speed limits on all the former 25 mph roads to 20 mph. The result is predictable. If the city wanted people to slow down on the small surface streets, they should make them narrow enough that it's somewhat challenging to fit two cars on side by side. Works every time, needs no more enforcement to make it work.