The total outside width of any vehicle or load thereon must not exceed eight and one-half feet; except that an externally mounted rear vision mirror may extend beyond the width limits of the vehicle body to a point that allows the driver a view to the rear of the vehicle along both sides in conformance with Federal National Safety Standard 111 (49 C.F.R. § 571.111), and RCW 46.37.400. Excluded from this calculation of width are certain devices that provide added safety, energy conservation, or are otherwise necessary, and are not designed or used to carry cargo. The width-exclusive devices must be identified in rules adopted by the department of transportation under RCW 46.44.101. A width-exclusive device must not extend more than three inches beyond the width limit of the vehicle body.