The maximum speed limits specified in section 39-09-02 may be altered on all or any part of the state highway system by an administrative order by the director after a public hearing has been held. Such determination must be based on engineering and traffic investigations with primary consideration given to the establishment of reasonable and safe speeds, highway    conditions, enforcement, and the general welfare. Speed limits established pursuant to this section shall be effective only when appropriate signs giving notice thereof are erected and such maximum speed limits may be declared to be effective at all times or at such times as are indicated upon said signs. Differing limits may be established for different times of the day, different types of vehicles, varying weather conditions, and other factors bearing on safe speeds, which shall be effective when posted upon appropriate fixed or variable signs.