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.