The Counterintuitive Truth About Roof Crush Standards

Ever since the United States began issuing safety and emission standards, regulations have led to better cars. Emission standards forced automakers to develop electronic engine controls, creating modern cars’ power and drivability. Safety standards– seat belts, airbags, etc.– have saved countless lives. But there’s one standard that’s not only ineffective, but antithetical to its stated goal: the National Highway Traffic Safety Administration’s (NHTSA) roof crush standard.

NHTSA enacted FMVSS 216 in the mid-70’s. Before then, unbelted passengers were often ejected during a crash and killed or injured by “external factors.” It was assumed that roof strength played an insignificant role during serious accidents. As seat belts gained popularity, NHSTA created its new roof crush standard to ensure that vehicle roofs wouldn’t crush belted occupants during a rollover.

FMVSS 216 requires all light vehicles sold in the United States to withstand a force equal to 1.5 times the vehicle’s unladen weight or 5000lbs. (whichever is less), with no more than five inches intrusion. NHTSA’s technicians apply a measured force to one side of the roof’s edge, at a 25-degree angle (sideways) and a five-degree pitch angle (forwards), then measure the deflection, or failure, of the roof structure. As a result of this requirement, car makers have strengthened their vehicles’ roof structures. It would be easy to posit that the standard was– and is– an excellent piece of federal legislation. A closer examination of the evidence suggests otherwise.

The legislation’s framers knew that convertibles would never be able to meet the new roof crush standard. So they exempted them. At the time that the standard was created, convertibles were becoming increasingly unpopular. NHTSA figured that extinction would make the exemption a moot point. When convertibles made a comeback, the exemption provided valuable statistical data. Unfortunately for the rule-makers, real world experience didn’t validate the roof crush standard’s original premise. Passengers in convertibles during rollover accidents were not killed in any greater numbers than occupants of cars that met NHTSA’s roof crush standard.

This empirical data has been ignored in the face of the increased lethality of rollover accidents. Although rollover crashes constitute just three percent of all American accidents, they account for almost half of all fatalities. That’s an estimated 26,376 deaths annually. But the assumption that a weak roof collapsed and crushed the passengers is simply not supported by the data. In 74% of cases, roof intrusion was not a factor. Rollover accidents are fatal because the occupants are usually ejected, or partially ejected, during the crash.

The best way to meet NHTSA’s roof crush standard: increase the strength (and weight) of components at the top of the car, well above the center of gravity. This adds significantly to an SUV’s “top-heavy” character, and contributes greatly to their propensity to roll when subjected to large lateral forces. In other words, the requirement to make roofs stronger to withstand rollovers may well be making vehicles more likely to rollover. NHTSA’s roof crush standard is creating a vicious circle; people may be dying because of the law– not despite it.

Safety campaigners like “Protecting Health, Safety & Democracy” are lobbying the federal government to revise roof crush standards “upwards.” They want roofs to be “stronger,” so they can withstand greater impacts. Although it’s theoretically possible to find light weight materials that would withstand insult better than steel, in the real world, more stringent legislation would result in even more weight up top. And that would mean that vehicles already more prone to rollover accidents will be made even more prone to rollover accidents by the increased stringency of the rollover protection standard.

In any case, data gathered over the last three decades shows that it’s time to repeal NHTSA’s roof crush standard. The increasing trend towards taller vehicles, with a greater propensity to roll over, argues for the repeal of this standard completely, before it does even more harm. While safety campaigners argue that the revised roof crush standards don’t go far enough, NHTSA should fully and scientifically investigate the likelihood that the exact opposite is true. The rule-makers must admit the possibility that they’ve made a mistake, and find a new and more effective fix for a deadly problem.