The bigger the vehicle, the safer it is in a crash, right? Not necessarily, say safety experts.

One of America’s two car safety testing agencies recently put three full-size SUVs through crash tests and found none worthy of its highest award.

America’s Two Crash Testing Agencies

Government car safety agencies worldwide put cars through crash tests. Only America has both a government and a non-government safety tester.

The federal National Highway Traffic Safety Administration (NHTSA) performs one set of tests, and the insurance industry performs another.

A group of car insurance companies funds its own lab, the Insurance Institute for Highway Safety (IIHS). The institute has a reputation as the stricter grader. NHTSA engineers must undergo a lengthy public approval process to strengthen their tests, but the IIHS can increase the difficulty without much bureaucratic process.

New Second-Row Test Dummy

They’ve done that recently, adding a dummy with the proportions of a 12-year-old child in the second row for a frontal crash test.

Automotive engineers tend to design cars to pass these tests. Changes like a new passenger can expose safety problems they haven’t aimed to solve.

Today’s cars protect front passengers with seatbelts, front and side airbags, and sometimes knee airbags and roofline airbags that protect their heads in a rollover. Rear passengers often get seatbelts and just one side airbag.

There may be good reasons they don’t get those same protections. Safety experts tell parents not to install child safety seats up front because airbags can dislodge them and leave them moving too much in a crash.

However, the new IIHS rear seat dummy has proven to be a problem for many cars. Midsize SUVs, midsize cars, small cars, large trucks, midsize trucks, luxury SUVs, and even kid-friendly minivans all saw their average test scores drop when the rear-seat dummy appeared.

Now, the largest SUVs have seen the same.

One of Three Earns High Marks

The IIHS says that the old logic that when two cars collide, you want to be in the bigger car is only partially right.  

“The huge mass of these large SUVs provides some additional protection in crashes with smaller vehicles, though that also means they present more danger to other road users,” says IIHS President David Harkey. “The flip side of their large size is that there is a lot more force to manage when they crash into a fixed obstacle like a tree or bridge abutment or the barrier we use in our front crash tests.”

The institute crashes cars into stationary barriers at 40 mph, offset slightly in the way a car would likely hit a turning car in an intersection. It awards scores of Good, Acceptable, Marginal, and Poor based on how well they protect the driver and a rear passenger.

None of the three large SUVs tested earned a score higher than marginal for protecting the child in the back.

Bad Scores Can Be Good for Safety

The news is discouraging for current car shoppers. But every time the institute makes its tests harder, safety engineers at automakers go to work figuring out how to pass the new tests.

“These discouraging results show that some popular vehicles still lag in meeting the most advanced safety standards,” says Raul Arbelaez, vice president of the institute’s Vehicle Research Center. “The good news is that the top performer in this class proves that automakers can readily address these problems.”

If history is any guide (it is), they’ll likely solve the rear-seat safety problem.

In the meantime, the rear seat is still the safest place for children. The adult-size airbags in front seats can injure them.