Most deaths in large truck crashes are passenger vehicle occupants. Large trucks often weigh 20-30 times as much as passenger vehicles. They are taller and have greater ground clearance than cars, which means that lower-riding vehicles can slide beneath truck trailers, with deadly consequences. Rear underride guards are supposed to stop this from happening, but IIHS research shows that guards meeting federal safety standards can fail in relatively low-speed crashes.

Truck driver fatigue is a known crash risk. Under federal hours-of-service regulations, drivers of large trucks are allowed to be behind the wheel for as long as 11 hours at a stretch. Surveys indicate that many drivers violate the regulations and work longer than permitted. Electronic logging devices, which will be required beginning in December 2017, should help with compliance.

Truck braking capability can be a factor in crashes. Compared with passenger vehicles, stopping distances for trucks are much longer, particularly on wet and slippery roads or if the brake systems are poorly maintained. Large trucks also are prone to rolling over. A requirement for electronic stability control took effect for most new truck tractors in 2017 and is expected to reduce crashes.

Rear underride tests

When cars hit the back of trucks, they sometimes slide underneath with often deadly results. Rear underride guards — metal bars that hang from the backs of trailers — are supposed to prevent this, but many guards don't hold up.

IIHS tests underride guards on trucks in three configurations. Trailer manufacturers whose guards pass all three tests earn our TOUGHGUARD award.