When local authorities may alter maximum limits.
(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 sixty miles per hour; or
(c) Decreases the limit but not to less than twenty 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 sixty miles per hour.
(3) 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.
(4) 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.
(5) 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.