The Federal Highway Administration has proposed criteria for excluding safety or efficiency enhancing devices from vehicle length and width measurements.

The federal government dictates the minimum allowable length and width of commercial trucks and buses on the National Network (Interstates and designated highways) but allows the states to exempt approved safety device devices, such as mirrors and turn signals, from length and weight measurements. The proposed rules, in the works since the early 1990s, would establish uniform criteria for allowable exemptions.
Under the proposal, the following would be excluded from length and weight measurements:
- All non-load-carrying devices that do not extend more than 3 inches beyond the front or each side of the vehicle, or 24 inches beyond the rear of the vehicle
- All non-load-carrying devices within the swing radius at the front of a semitrailer or trailer. Swing radius is defined as the volume bounded by the front wall of a semitrailer or trailer and the arc formed when a line centered on the kingpin is rotated from the lower left to the lower right front corner of the vehicle. The swing radius extends from the bottom to the top of the semitrailer or trailer.
- Resilient bumpers extending up to 6 inches from the front and rear of a vehicle.
- Rear view mirrors that extend up to 12 inches and turn signal lamps that extend up to 6 inches beyond each side of a vehicle.
- Aerodynamic devices made of flexible material that are inflated by air pressure and lack a rigid structure if they extend no more than 8 feet beyond the rear of a vehicle and if they don’t obscure tail lamps, turn signals, marker lamps, identification lamps, license plates or any other required safety devices such as hazardous materials placards or conspicuity markings.
- A front overhang of up to 3 feet and rear overhang of up to 4 feet on automobile transporters. Extendable ramps or “flippers” used to achieve the allowable 3 foot front and 4-foot rear overhangs are excluded from length measurements if they are retracted when not supporting cargo.
Vehicle components not excluded by law or regulation would be included in measurements.
The proposed rules, with discussion and industry comments generated by an earlier comment request, were published in the August 18 Federal Register which can be accessed on the Internet at Comments are due November 16 and can be submitted electronically at Refer to FHWA Docket No. 1997-2234.