(1) Whenever the Department of Transportation determines, upon the basis of an engineering and traffic investigation, that any maximum speed limit is greater or less than is reasonable or safe under the conditions found to exist at any intersection, place, or part of the state highway system outside of the corporate limits of cities and villages as well as inside the corporate limits of cities and villages on freeways which are part of the state highway system, it may determine and set a reasonable and safe maximum speed limit for such intersection, place, or part of such highway which shall be the lawful speed limit when appropriate signs giving notice thereof are erected at such intersection, place, or part of the highway, except that the maximum rural and freeway limits shall not be exceeded. Such a maximum speed limit may be set to be effective at all times or at such times as are indicated upon such signs.

(2) The speed limits set by the department shall not be a departmental rule, regulation, or order subject to the statutory procedures for such rules, regulations, or orders but shall be an authorization over the signature of the Director-State Engineer and shall be maintained on permanent file at the headquarters of the department. Certified copies of such authorizations shall be available from the department at a reasonable cost for duplication. Any change to such an authorization shall be made by a new authorization which cancels the previous authorization and establishes the new limit, but the new limit shall not become effective until signs showing the new limit are erected as provided in subsection (1) of this section.

(3) On county highways which are not part of the state highway system or within the limits of any state institution or any area under control of the Game and Parks Commission or a natural resources district and which are outside of the corporate limits of cities and villages, county boards shall have the same power and duty to alter the maximum speed limits as the department if the change is based on an engineering and traffic investigation comparable to that made by the department. The limit outside of a business or residential district shall not be decreased to less than thirty-five miles per hour.

(4) On all highways within their corporate limits, except on state-maintained freeways which are part of the state highway system, incorporated cities and villages shall have the same power and duty to alter the maximum speed limits as the department if the change is based on engineering and traffic investigation, except that no imposition of speed limits on highways which are part of the state highway system in cities and villages under forty thousand inhabitants as determined by the most recent federal decennial census or the most recent revised certified count by the United States Bureau of the Census shall be effective without the approval of the department.

(5) The director of any state institution, the Game and Parks Commission, or a natural resources district, with regard to highways which are not a part of the state highway system, which are within the limits of such institution or area under Game and Parks Commission or natural resources district control, and which are outside the limits of any incorporated city or village, shall have the same power and duty to alter the maximum speed limits as the department if the change is based on an engineering and traffic investigation comparable to that made by the department.

(6) Not more than six such speed limits shall be set per mile along a highway, except in the case of reduced limits at intersections. The difference between adjacent speed limits along a highway shall not be reduced by more than twenty miles per hour, and there shall be no limit on the difference between adjacent speed limits for increasing speed limits along a highway.

(7) When the department or a local authority determines by an investigation that certain vehicles in addition to those specified in sections 60-6,187, 60-6,305, and 60-6,313 cannot with safety travel at the speeds provided in sections 60-6,186, 60-6,187, 60-6,189, 60-6,305, and 60-6,313 or set pursuant to this section or section 60-6,188 or 60-6,189, the department or local authority may restrict the speed limit for such vehicles on highways under its respective jurisdiction and post proper and adequate signs.