Section 39-09-03 - When local authorities may or shall alter maximum speed - Limits - Signs posted1. Whenever local authorities in their respective jurisdictions, on the basis of an engineering and traffic investigation, determine that the maximum speed permitted under this title is greater or less than is reasonable and safe under the conditions found to exist upon a highway or part of a highway, the local authority may determine and declare a reasonable and safe maximum limit thereon which: a. Decreases the limit at intersections;b. Increases the limit within an urban district but not to more than fifty-five miles [88.51 kilometers] per hour; orc. Decreases the limit outside an urban district.2. Local authorities in their respective jurisdictions shall determine by an engineering and traffic investigation the proper maximum speed for arterial streets and shall declare a reasonable and safe maximum limit thereon which may be greater or less than the maximum speed permitted under this chapter for an urban district.3. Any altered limit established as hereinabove authorized shall be effective at all times or during hours of darkness or at other times as may be determined when appropriate signs giving notice thereof are erected upon such street or highway.4. Any alteration of maximum limits on state highways or extensions thereof in a municipality by local authorities may not be effective until such alteration has been approved by the director.5. Not more than six such alterations as hereinabove authorized may be made per mile [1.61 kilometers] along a street or highway, except in the case of reduced limits at intersections, and the difference between adjacent limits may not be more than ten miles [16.09 kilometers] per hour.