(1) Tribal authorities, within their reservation boundaries, may determine based on an engineering and traffic investigation that the maximum speed permitted under RCW 46.61.400 or 46.61.405 is greater or less than is reasonable or safe under the conditions found to exist upon a nonlimited access state highway or part of a nonlimited access state highway. Then, the tribal authority may determine and declare a reasonable and safe maximum limit thereon which:

(a) Decreases the limit at intersections;

(b) Increases the limit, not exceeding sixty miles per hour; or

(c) Decreases the limit, not lower than twenty miles per hour.

(2) Any alteration by tribal authorities of maximum limits on a nonlimited access state highway is not effective until the alteration has been approved by the secretary of transportation and appropriate signs giving notice of the alteration have been posted. In the case of an alteration by tribal authorities of maximum limits on a nonlimited access state highway that is also part of a city or town street or county road within tribal reservation boundaries, the alteration is not effective until that alteration has also been approved by the applicable local authority.