Roof-Strength Ratings Offer Insight on Rollover Safety

For the 2010 model year, the Insurance Institute for Highway Safety implemented the first standardized test to reflect how vehicles and their occupants might fare in a rollover. By considering a particular model's weight versus its roof strength, as determined by a real crush test, the organization now rates vehicles using its familiar Good/Acceptable/Marginal/Poor rating scale.

Previously, all rollover ratings had focused on two things: the likelihood that a particular model would roll over, and active-safety features like electronic stability systems and Ford's Roll Stability Control that attempt to prevent a pending rollover. Some models also recognize when a rollover is occurring and deploy side curtain airbags, which can protect occupants and prevent their ejection. But a measure of the cabin's ability to maintain its shape had never before been rated.

How It Works

In the IIHS test, a metal plate applies pressure to one side of the test subject's roof until the roof intrudes 5 inches into the passenger compartment. The amount of weight required to reach this point is one factor in the calculation. The second factor is the weight of the vehicle itself, which is the force acting against the roof in a rollover. If the roof withstands at least four times the vehicle's weight, it earns a Good rating. The strength-to-weight ratio for an Acceptable rating is 3.25, and the minimum for a Marginal rating is 2.5. Anything below that ratio earns a Poor rating.

According to the National Highway Traffic Safety Administration, a majority of fatalities in rollover accidents happen when occupants aren't wearing seat belts and/or they are ejected from the car. A strong roof can't make people wear seat belts, but IIHS says it can prevent windows from breaking and doors from opening, which reduces the chance of ejections during a rollover. IIHS reports that a study of 12 small-car models showed a 39 percent lower ejection risk in the cars with the strongest roofs, and a 35 percent reduction in serious injuries or fatalities.

Some Hybrids Score Lower

So far, the ratings run the gamut for 2010 and 2011 models tested, with no predictability in terms of how a particular model or vehicle class might fare in the test. Rollovers tend to be single-vehicle accidents, so the vehicle's size isn't an issue — only its weight relative to its roof strength.

One trend is that hybrid versions of gas-only models can earn lower ratings because hybrid hardware and batteries add significant weight to the car. For example, the Ford Escape is rated Marginal, but the Escape Hybrid is rated Poor. Though it's unlikely, features like four-wheel drive — which adds weight — could also result in two ratings for the same model, IIHS spokesman Russ Rader said. IIHS can do the calculation based on any car's weight once the roof-strength data are collected.

Roof Strength and Top Safety Picks

Starting with the 2010 model year, roof strength joined the criteria for IIHS' Top Safety Pick designation, which requires a model to earn Good ratings in all the organization's tests: frontal-offset, side-impact and rear-crash protection. In the past, model-year changes allowed automakers to improve their rear-crash ratings, quickly increasing the winners' ranks. The new requirement stripped the citation from many models, making Top Safety Picks a more exclusive group.

© Cars.com 05/10/2013