Rear Impact Guards and Underride Protection Standards

The Underride Problem

Rear underride crashes occur when a passenger vehicle strikes the rear of a trailer or semitrailer and slides underneath the cargo area. These collisions are among the most deadly types of truck-involved crashes because the passenger vehicle's safety systems -- crumple zones, airbags, and seatbelts -- are bypassed when the vehicle structure passes beneath the trailer's rear. Rear impact guards are designed to prevent this by providing a barrier that engages the crashing vehicle's front-end safety structures.

Federal Standards: FMVSS 223 and 224

Two Federal Motor Vehicle Safety Standards (FMVSS) govern rear impact guards:

FMVSS 223: Rear Impact Guards

This standard establishes the strength and energy absorption requirements for rear impact guards themselves. It specifies the minimum performance criteria that a guard must meet, including resistance to static force loads applied at specific points across the guard's width. Guards must be able to withstand prescribed force levels without excessive deflection to ensure they stop a colliding vehicle effectively.

FMVSS 224: Rear Impact Protection

FMVSS 224 requires that most trailers and semitrailers with a gross vehicle weight rating (GVWR) of 10,000 pounds or more be equipped with a rear impact guard that meets FMVSS 223 performance requirements. The guard must be installed so that the bottom edge is no more than 22 inches above the ground (measured on an unloaded trailer) and must extend to within 4 inches of the side extremity of the trailer on each side.

Exemptions

Certain vehicle types are exempt from FMVSS 224, including:

• Wheels-back vehicles where the rear axle is at the extreme rear of the vehicle
• Low-chassis trailers with the main cargo floor less than 22 inches above the ground
• Pole trailers
• Pulpwood and other specialty trailers designed for specific cargo configurations
• Temporary equipment such as road construction machinery

FMCSR Requirements for In-Service Vehicles

While FMVSS standards apply to manufacturers at the time of production, the Federal Motor Carrier Safety Regulations (FMCSRs) impose ongoing requirements on carriers operating these vehicles. Under 49 CFR 393.86, trailers and semitrailers manufactured after January 26, 1998, must maintain their rear impact guards in proper working condition throughout the vehicle's service life.

During roadside inspections, officers check rear impact guards for:

• Missing guard -- the guard is completely absent on a trailer that requires one
• Damaged or deformed guard -- the guard shows evidence of impact damage, corrosion, or structural compromise that would reduce its effectiveness
• Improper mounting -- the guard is not securely attached or has loose or missing fasteners
• Ground clearance -- the bottom edge exceeds the maximum allowable height from the road surface
• Width coverage -- the guard does not extend close enough to the trailer's side edges

Violations and Enforcement

Rear impact guard violations fall under the Vehicle Maintenance BASIC in the Safety Measurement System. While individual guard violations may carry moderate severity weights, they are often found alongside other vehicle maintenance deficiencies during inspections, compounding the impact on a carrier's BASIC percentile.

Severe guard deficiencies -- such as a completely missing guard on a loaded trailer -- can result in an out-of-service order, requiring the vehicle to be parked until the deficiency is corrected. Carriers should include rear impact guard inspection as part of their regular preventive maintenance program and driver pre-trip inspection routine.

Look up specific rear impact guard violation codes and their severity weights on TruckCodes.