Speed limits are the most common method for managing speed.  The current framework for setting speed limits was developed in the 1920s and 1930s.  Each state has a basic rule that requires drivers to operate vehicles at a speed that is reasonable and prudent for existing environmental conditions.  State statutes specify speed limits that generally apply to different road types or geographic areas.  However, State and most local governments have the authority to set speed limits on the basis of an engineering study by establishing speed zones for highway sections where statutory limits do not fit specific road or traffic conditions.

Speed limits in speed zones are established for favorable conditions -- good weather, free-flowing traffic, and good visibility.  Drivers are expected to reduce speeds as conditions deteriorate.  The most common approach sets the limit on the basis of an engineering study, which takes into consideration such factors as operating speeds of free-flowing vehicles, crash experience, roadside development, roadway geometry, and parking and pedestrian activity levels to make a judgment about the speed at which the posted limit should be set.  However, pressure from the public or from elected officials to lower speed limits is common and hard to resist when procedures for setting speed limits are seen as subjective and not well understood.