Originally Posted by PaulR
It is very scientific. Speed limits are set just as other rates. Calculations are made using base averages. In the case of speeds the road conditions and environment are taken into consideration. The biggest factor, however is the reaction time of the AVERAGE driver to respond to an incident in the given conditions and environment. Long ago it was found the average driver in a metro area cannot react to distractions and other factors during heavy traffic such as rush hour. In the 70's when the speed limits were lowered from 75 to 60 the accident rates went way down in such scenarios. So did the number of fatalities.
This is true when they are properly set, but not when they are set by the preferences (some might say "whims") of local politicos or law enforcement. And, we all know that in some communities, speed limits are revenue generation opportunities. FWIW, the Boulder courts had never heard of the 85th percentile (Manual On Uniform Traffic Control Devices), although I believe that traffic engineering had.
Also, this information on accidents is inaccurate. The real issue is natural traffic flow for a given road. The Martin Parker Report
(Federal Highway Administration 1997) shows that speed limits do not actually effect travel speeds (+/- 1.5 mph), but reducing speed limits increases
accidents! When limits went up, crashes went down. The issue is speed conflicts between vehicles.
Just recently, Texas raised the speed limit on I-10 and I-20 in west Texas to 80 mph. From 1999-2001 when the limit was 70 mph, there were 92 fatalities. Raising it to 75 dropped them 13 percent to 80 from 2002-2004. 85th percentile speed on those roads is 79 mph, which led to the speed limit increase.