As of 2015, street speed limits are set by state and local governments by statute. Furthermore, every state has a basic speed law, requiring drivers to maintain a reasonable and prudent speed for the conditions, according to the U.S. Department of Transportation Federal Highway Administration.
State speed limits are often derived from the Uniform Vehicle Code, a model set of laws created to encourage vehicle code uniformity among the states. There are four general methods used for setting speed limits, according to the Federal Highway Administration. First, there is the engineering approach, which starts by setting a base speed limit at the 85th percentile of free-flowing traffic, and then adjusting this number based on road conditions. Second, there is the expert-system approach, where computers set a speed limit by simulating the behavior of cars using speed-limit experts' knowledge.
Third, there is the optimization approach, which sets speed limits based on the total societal costs of transportation, including noise, pollution levels, crashes and travel time, notes the Federal Highway Administration. Fourth, there is the injury-minimization approach, where speed limits are set according to crash types that typically occur on a road, the corresponding impact forces and the typical human body's response to these impact forces. States and local municipalities often use one or more of the above methods in combination to set jurisdictional speed limits.