A city’s ability to change speed limits is impacted by rules and practices around enforcement, signage, and design requirements.
A city’s ability to enforce the posted speed limit depends on whether speed limits in the state are Absolute, Prima Facie, Basic Speed Law, or a combination of the three. When drivers are ticketed in a state with absolute speed limits, the ticket will typically stand on face value. In states with prima facie, or presumed, speed limits, drivers can contest tickets in court on the basis that their speed was safe for the conditions. In basic speed law states, drivers are required only to travel at a safe speed, regardless of the posted speed limit. A growing body of evidence shows that drivers respond to posted speed limits even without changes to enforcement; cities may want to make speed limit changes even when enforcement is difficult.
In some states, cities must implement physical design changes to streets in order to justify lower speed limits. Requiring engineering changes before cities can change the posted limit can make it difficult for cities to change speed limits on a large number of streets because of the cost. Other cities must reduce speed limits before they can make design changes, since the design speed is set in relation to the posted speed on a given street.
In some states, a city must install a sign on every block if the posted speed limit is anything other than the citywide default. This is feasible when the city lowers the limit on a small number of segments, but becomes prohibitively expensive at a large scale (e.g., across all residential streets).