(1) Whenever local authorities in their respective jurisdictions determine on the basis of an engineering and traffic investigation that the maximum speed permitted under RCW
46.61.400 or
46.61.440 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; or
(b) Increases the limit but not to more than 60 miles per hour; or
(c) Decreases the limit but not to less than 20 miles per hour.
(2) Local authorities in their respective jurisdictions shall determine by an engineering and traffic investigation the proper maximum speed for all arterial streets and shall declare a reasonable and safe maximum limit thereon which may be greater or less than the maximum speed permitted under RCW
46.61.400(2) but shall not exceed 60 miles per hour.
(3)(a) Local authorities in their respective jurisdictions may establish a maximum speed limit of 20 miles per hour on a nonarterial highway or part of a nonarterial highway.
(b) A speed limit established under this subsection by a local authority does not need to be determined on the basis of an engineering and traffic investigation if the local authority has developed procedures regarding establishing a maximum speed limit under this subsection. Any speed limit established under this subsection may be canceled within one year of its establishment, and the previous speed limit reestablished, without an engineering and traffic investigation. This subsection does not otherwise affect the requirement that local authorities conduct an engineering and traffic investigation to determine whether to increase speed limits.
(c) When establishing speed limits under this subsection, local authorities shall consult the manual on uniform traffic control devices as adopted by the Washington state department of transportation.
(4) The secretary of transportation is authorized to establish speed limits on county roads and city and town streets as shall be necessary to conform with any federal requirements which are a prescribed condition for the allocation of federal funds to the state.
(5) Any altered limit established as hereinbefore authorized shall be effective when appropriate signs giving notice thereof are erected. Such maximum speed limit may be declared to be effective at all times or at such times as are indicated upon such 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 when posted upon appropriate fixed or variable signs.
(6) Any alteration of maximum limits on state highways within incorporated cities or towns by local authorities shall not be effective until such alteration has been approved by the secretary of transportation.