What Is a Protector for Truck? | Side Guards Explained

A protector for truck is a safety device mounted along the side of a large vehicle, most often a trailer or semitrailer. Its job is to close the dangerous gap between the front and rear wheels so that a person on foot, a bicycle, or a motorcycle is deflected instead of pulled beneath the wheels in a collision. While the term “side guard” is the common shorthand, federal transportation researchers classify these as lateral protective devices, and they are a different piece of equipment from the rear underride guards mounted at the back of a trailer.

What Exactly Does a Lateral Protective Device Do?

A lateral protective device physically covers the exposed open space beside a truck’s wheels, creating a barrier that blocks vulnerable road users from entering the under-vehicle area during a side-impact crash. The United States Department of Transportation’s Volpe Center describes the purpose as reducing the risk of run-over and severe injury to pedestrians, bicyclists, and motorcyclists.

These guards are designed to withstand crash force, but the intent is not simply to absorb an impact. The device redirects a person or bicycle away from the wheels, preventing the most catastrophic outcome. The concept has been proven internationally; a DOT literature review notes that lateral protective devices have been required on trucks in Europe since 1989, and similar requirements exist in at least 32 countries for certain motor vehicles, trailers, and semitrailers.

Side Guards vs. Underride Guards: What’s the Difference?

A common point of confusion is treating side guards and underride guards as the same thing. They are not.

• Lateral protective devices (side guards): Mounted along the side of the vehicle, between the front and rear wheels, to protect vulnerable road users from being swept underneath.
• Rear underride guards: Mounted at the back of a trailer to prevent a smaller vehicle from sliding beneath the trailer in a rear-end collision.
• Side underride guards: A related concept addressed by the National Highway Traffic Safety Administration (NHTSA), focused specifically on preventing passenger vehicles from underriding trailers and semitrailers from the side.

NHTSA’s June 2024 Report to Congress on side underride protection, prepared under the Infrastructure Investment and Jobs Act, examines the side underride guard issue for trailers and semitrailers. The Massachusetts state guidance is equally explicit: “lateral protective devices” and “side guards” refer to the same piece of equipment, and they are not rear impact guards.

Are Protectors for Trucks Required in the US?

There is currently no federal mandate requiring lateral protective devices on all trucks operating in the United States. The existing federal rulemaking from the National Highway Traffic Safety Administration focuses on rear impact guards, not side guards. However, the regulatory landscape is shifting at the state level, with Massachusetts leading the way.

Massachusetts requires certain trucks and trailers to be equipped with lateral protective devices. The requirement applies to trailers with a gross vehicle weight rating (GVWR) over 10,000 pounds that are subject to the state’s truck safety device law. The state also mandates convex mirrors, cross-over mirrors, and back-up cameras as part of the compliance package.

There are exemptions. The Massachusetts law does not apply to ambulances, other emergency medical vehicles, firefighting vehicles, low-speed vehicles, agricultural tractors, certain Commonwealth-owned vehicles leased or purchased before January 1, 2023, and certain emergency-contract vehicles. Owners of vehicles subject to the law must certify that their vehicles are properly equipped.

Where to Find a Protector for Your Truck

The distinction between side guards and rear guards matters when you’re shopping. A rear underride guard won’t protect a cyclist riding alongside your trailer, and a side guard won’t stop a car from sliding under your trailer in a rear-end collision. Before you buy, know which protection your vehicle actually needs based on its configuration and your operating region.

For fleet owners and operators looking to equip trailers with side protection, our tested roundup of the best truck lateral protection options breaks down the top products by durability, ease of installation, and crash performance. The review covers the practical details that matter on the road, from mounting hardware to weight considerations.

For technical reference, the Federal Motor Carrier Safety Administration’s research on lateral protective devices provides the engineering background on how these guards perform in real-world crash scenarios.