(1) Whenever the secretary of transportation shall determine upon the basis of an engineering and traffic investigation that any maximum speed hereinbefore set forth is greater than is reasonable or safe with respect to a state highway under the conditions found to exist at any intersection or upon any other part of the state highway system or at state ferry terminals, or that a general reduction of any maximum speed set forth in RCW
46.61.400 is necessary in order to comply with a national maximum speed limit, the secretary may determine and declare a reasonable and safe lower maximum limit or a lower maximum limit which will comply with a national maximum speed limit, for any state highway, the entire state highway system, or any portion thereof, which shall be effective when appropriate signs giving notice thereof are erected. The secretary may also fix and regulate the speed of vehicles on any state highway within the maximum speed limit allowed by this chapter for special occasions including, but not limited to, local parades and other special events. Any such maximum speed limit may be declared to be effective at all times or at such times as are indicated upon the said signs; and differing limits may be established for different times of day, different types of vehicles, varying weather conditions, and other factors bearing on safe speeds, which shall be effective (a) when posted upon appropriate fixed or variable signs or (b) if a maximum limit is established for auto stages which is lower than the limit for automobiles, the auto stage speed limit shall become effective 30 days after written notice thereof is mailed in the manner provided in RCW
46.61.410(4), as now or hereafter amended.
(2) The secretary of transportation may establish a maximum speed limit of 20 miles per hour on a nonarterial state highway, or part of a nonarterial state highway, without a determination made on the basis of an engineering and traffic investigation, subject to the conditions described in RCW
46.61.415(3).