A common belief is that posting a speed limit will influence drivers to drive at that speed. The facts indicate otherwise.
Research conducted in many parts of this country over a span of several decades has shown that drives are influenced more by the appearance of the highway itself and the prevailing traffic conditions than by posted speed limit.
Minnesota’s Basic Speed Law requires that:

“No person shall drive a vehicle on a highway at a speed greater than is reasonable and prudent under the conditions and having regard to the actual and potential hazards then existing. In every event speed shall be so restricted as may be necessary to avoid colliding with any person, vehicle or other conveyance on or entering the highway in compliance with legal requirements and the duty of all persons to use due care.”

In Minnesota, the maximum speed limit in an urban district is 30 miles per hour unless otherwise posted. An urban district is defined as the territory contiguous to and including any street which is built up with structures devoted to business, industry, or dwelling houses situated at intervals of less than 100 feet for a distance of a quarter of a mile or more. Outside urban districts, the maximum speed limit for any passenger vehicle is currently 55 miles per hour. These speeds are not always posted but all Minnesota motorists are required to know these basic 30 and 55 mile per hour speed laws. Note that freeways and expressways may be posted between 60 and 70 mph depending upon conditions.
Under Minnesota law, intermediate speed limits (except school speed limits) between 30 and 55 miles per hour may be established on any road, including County highways and City streets, only by the State Commissioner of Transportation. The commissioner must establish the speed limit upon the basis of an engineering and traffic investigation. This investigation includes an analysis of roadway conditions, accident records and the prevailing speed of prudent drivers. If speed limit signs are posted for a lower limit than is needed to safely meet these conditions, many drivers will simply ignore the signs. At the same time, other drivers will stay within the posted limits. This generally increases the conflicts between faster and slower drivers, reduces the gaps in traffic through which crossings could be made safely and increases the difficulty for pedestrians to judge the speed of approaching vehicles. Studies have shown that where uniformity of speeds is not maintained, accidents generally increase.