Although it is a commonly held belief that posting a speed limit sign will reduce speeds, research indicates that a driver's speed is influenced more by their comfort level driving on the road, which can be commonly related to the geometrics of the road and the prevailing traffic conditions rather than by the posted speed limit.
Additionally, if speed limit signs are posted artificially low, many drivers will ignore posted limits while some drivers will stay within the posted limits. This creates a conflict between faster and slower moving drivers and an increase in the number of accidents.
New laws, effective August 1, 2019, amended how speed limits are set in the state of Minnesota. Essentially there are two ways the speed limits are set in Minnesota.
The first method, like most states, is from state law which establishes some speed limits (statutory speed limits). Here is a short list:
For the long list including details, see Minnesota Statutes: Chapter 169
The second way to set speed limits is by an engineering study. There are several methods, and they all use information about driver behavior (how fast people are driving and how often crashes occur), the roadway (straight vs. curved, up and down vs. flat) and the roadside (fixed objects close to the road, steep slopes). The Commissioner of MnDOT holds the authority to establish speed limits on nearly all roads in Minnesota. This unified authority allows for a uniform approach for establishing speed limits. There are several ways an agency can establish its own speed limits, most notably a city may set a speed limit after it meets a specific set of criteria on roads it has jurisdiction over.
Ultimately, speed limits are maximums under ideal conditions. State law requires that "No person shall drive a vehicle at a speed greater than is reasonable in regard to weather, visibility, traffic and the surface width of the roadway. Under no circumstances shall a person operate a vehicle at a speed that endangers the safety of persons or property."