The physics are brutal. A passenger car rear-ends a tractor-trailer at highway speed. Instead of bumpers meeting, the car slides underneath. The trailer’s undercarriage shears through the passenger compartment at windshield height—exactly where occupants sit. Airbags and crumple zones become irrelevant when impact occurs above them. Families traveling home, commuters heading to work, parents picking up children—all facing catastrophic injuries because of a protection gap recognized for decades.
Below, our friends at Warner & Fitzmartin – Personal Injury Lawyers explain underride accidents.
What Makes Underride Crashes So Deadly
Underride accidents occur when height differences allow a car to slide under a truck. The gap between a truck’s rear wheels and the road can be several feet—enough for a sedan’s hood to fit underneath. The trailer intrudes directly into the passenger space, often causing traumatic brain injuries, spinal cord damage, and severe facial trauma.
According to the Government Accountability Office, an average of about 219 fatalities from underride crashes were reported annually from 2008 through 2017. More recent analysis by FRONTLINE and ProPublica found that more than 400 people died in underride crashes in 2021 alone. These numbers are widely considered underreported because police officers often don’t properly document underride crashes.
The Insurance Institute for Highway Safety estimates that underride occurs in approximately half of all fatal crashes between large trucks and passenger vehicles. Of these, 57 percent involve the truck’s front, 22 percent the rear, and 20 percent the side.
Three Types Of Underride Crashes
Rear underride happens when a vehicle strikes the back of a truck and slides underneath. Common scenarios include trucks stopping suddenly on highways, trailers parked without adequate warning lights, or nighttime visibility issues. The car’s front end goes under the trailer, bringing the trailer bed into direct contact with the windshield and roof.
Side underride occurs when a vehicle crashes into a trailer’s side and slides beneath it. These accidents often happen when trucks make turns across traffic, execute nighttime U-turns, or merge into occupied lanes. The car wedges underneath between the front and rear axles.
Front underride involves a truck driving over a smaller vehicle. While less common, these crashes occur at intersections or when trucks fail to maintain adequate following distance.
Why Standard Safety Features Fail
Standard safety systems in passenger vehicles aren’t designed for underride scenarios. Crumple zones absorb impact through controlled deformation of front or rear structures. In underride crashes, impact occurs above these zones, bypassing the strongest frame parts.
Airbags deploy based on sensors at specific vehicle points. Initial underride contact may not trigger sensors appropriately. Even when airbags deploy, they protect against dashboard impacts—not trailer beds intruding at head height.
Seatbelts restrain occupants when the passenger compartment stays intact. During underride, the trailer can crush or shear off the vehicle’s upper portion. Seatbelts can’t protect against intrusion into the occupant space itself.
The Underride Guard Problem
Federal regulations require tractor-trailers to have rear underride guards—steel bars designed to prevent vehicles from sliding underneath. These became mandatory in 1998 under Federal Motor Vehicle Safety Standards 223 and 224. Trucking companies and trailer manufacturers must comply with these standards, yet the standards themselves have proven inadequate.
Crash testing reveals significant weaknesses. The Insurance Institute for Highway Safety began testing guards in 2011 and found many failed, particularly in offset crashes. Even at 35 mph, severe underride occurred when guards couldn’t withstand impact. This raises questions about whether manufacturers have done enough when better designs exist.
IIHS introduced its “TOUGHGUARD” award in 2017 for trailers preventing underride in full-width, 50 percent overlap, and 30 percent overlap crashes. Many standard guards fail these tests—meeting legal requirements but not adequately protecting vehicles in real scenarios.
Federal regulations don’t require side underride guards, despite research showing they could prevent significant fatalities. IIHS estimates that side guard mandates could prevent approximately 160 to 220 deaths annually. Guards tested at 40 mph successfully prevented underride, but regulatory requirements haven’t caught up. There are also no federal requirements for front guards, though the European Union has required them since 2003.
Inspection And Maintenance Gaps
Even with rear guards installed, they may not function properly. Rust, loading dock damage, and wear can compromise structural integrity. Yet annual commercial vehicle inspections don’t specifically require guard examination. A truck accident lawyer knows that trucking companies have responsibility for maintaining their equipment, but damaged guards may look intact while being structurally compromised. When a guard fails during impact, questions arise about whether proper maintenance was performed and documented.
Underreporting The Problem
Accurate data on underride crashes is lacking. Police officers don’t use standardized definitions of underride crashes, and many haven’t received identification training. State crash report forms vary—some don’t include specific underride fields. When underride isn’t properly documented, national databases undercount these fatalities, making it difficult to justify new safety requirements.
The Bottom Line
Underride crashes represent one of the most devastating types of truck accidents because they defeat every safety system designed to protect occupants. The height mismatch creates a gap allowing trailers to intrude directly into passenger compartments, causing catastrophic injuries that standard vehicle safety features cannot prevent.
While rear underride guards have been federally mandated since 1998, crash testing shows that many guards meeting legal standards still fail in real-world collisions. Side guards could prevent hundreds of deaths annually, yet they remain optional despite proven effectiveness. When severe injuries or deaths occur in underride crashes, investigations often reveal that better protection existed but wasn’t required—or that required protection wasn’t properly maintained. The gap between legal requirements and actual effectiveness continues to exact a human cost that affected families understand all too well.