The Agency and the selectmen of the town where a covered bridge is located or, if parts of such a bridge are located in more than one town, the selectmen of the towns acting jointly, may restrict the use of the bridge to vehicles which are within limits as to weight, height, and width as they shall establish. The limitation shall be plainly posted at the approaches to the bridge at approximately 100 feet from each end of the bridge, and at intersections as may be required to enable operators of restricted vehicles to proceed by the most direct alternate unrestricted route. Posting shall be by means of permanent signs of a standard size of at least 24 inches by 24 inches, and with lettering not less than three inches high. (Added 1985, No. 269 (Adj. Sess.), § 1.)