This final rule upgrades NHTSA's safety standards addressing rear underride protection in crashes of passenger vehicles into trailers and semitrailers by adopting similar requirements to Transport Canada's standard for rear impact guards. Adopting these standards will require rear impact guards to provide sufficient strength and energy absorption to protect occupants of compact and subcompact passenger cars impacting the rear of trailers at 56 kilometers per hour (km/h) (35 miles per hour (mph)). Upgraded protection will be provided in crashes in which the passenger motor vehicle hits: the center of the rear of the trailer or semitrailer; and, in which 50 percent of the width of the passenger motor vehicle overlaps the rear of the trailer or semitrailer. This rulemaking commenced in response to petitions for rulemaking from the Insurance Institute for Highway Safety (IIHS) and from Ms. Marianne Karth and the Truck Safety Coalition (TSC). This final rule responds to and fulfills the rulemaking mandate of the November 2021 Bipartisan Infrastructure Law (BIL) that directs the Secretary to upgrade current Federal safety standards for rear impact guards. NHTSA is also issuing this final rule pursuant to DOT's January 2022 National Roadway Safety Strategy, which describes the five key objectives of the Department's Safe System Approach: safer people, safer roads, safer vehicles, safer speeds, and post-crash care. One of the key Departmental actions to enable safer vehicles is to issue a final rule to upgrade existing requirements for rear impact guards on newly manufactured trailers and semitrailers.