American IIHS says midsizers safer than small cars

For years, sub-compact cars have been shunned by snooty people who chose to drive a big car, citing concerns over safety. While their reasons probably have more to do with image than safety, the Insurance institute for Highway Safety (IIHS), an independent American agency that crash-tests hundreds of cars every year, say that the snooty people were actually right. While many sub-compact cars such as the Honda Jazz and the Toyota Yaris scored full marks in standardised crash tests against a stationary barrier, their downfalls became obvious when they were slammed against a midsize Honda Accord or a Toyota Camry.

Three front-to-front crash tests, each involving a microcar or minicar into a midsize model from the same manufacturer, show how extra vehicle size and weight enhance occupant protection in collisions. These Insurance Institute for Highway Safety tests are about the physics of car crashes, which dictate that very small cars generally can’t protect people in crashes as well as bigger, heavier models.

“There are good reasons people buy minicars,” says Institute president Adrian Lund. “They’re more affordable, and they use less gas. But the safety trade-offs are clear from our new tests. Equally clear are the implications when it comes to fuel economy. If automakers downsize cars so their fleets use less fuel, occupant safety will be compromised. However, there are ways to serve fuel economy and safety at the same time.”

The Institute didn’t choose SUVs or pickup trucks, or even large cars, to pair with the micro and minis in the new crash tests. The choice of midsize cars reveals how much influence some extra size and weight can have on crash outcomes. The Institute chose pairs of 2009 models from Daimler, Honda, and Toyota because these automakers have micro and mini models that earn good frontal crashworthiness ratings, based on the Institute’s offset test into a deformable barrier. Researchers rated performance in the 40 mph car-to-car tests, like the front-into-barrier tests, based on measured intrusion into the occupant compartment, forces recorded on the driver dummy, and movement of the dummy during the impact.

Laws of physics prevail: The Honda Jazz, Smart Fortwo, and Toyota Yaris are good performers in the Institute’s frontal offset barrier test, but all three are poor performers in the frontal collisions with midsize cars. These results reflect the laws of the physical universe, specifically principles related to force and distance.

Although the physics of frontal car crashes usually are described in terms of what happens to the vehicles, injuries depend on the forces that act on the occupants, and these forces are affected by two key physical factors. One is the weight of a crashing vehicle, which determines how much its velocity will change during impact. The greater the change, the greater the forces on the people inside and the higher the injury risk. The second factor is vehicle size, specifically the distance from the front of a vehicle to its occupant compartment. The longer this is, the lower the forces on the occupants.

Size and weight affect injury likelihood in all kinds of crashes. In a collision involving two vehicles that differ in size and weight, the people in the smaller, lighter vehicle will be at a disadvantage. The bigger, heavier vehicle will push the smaller, lighter one backward during the impact. This means there will be less force on the occupants of the heavier vehicle and more on the people in the lighter vehicle. Greater force means greater risk, so the likelihood of injury goes up in the smaller, lighter vehicle.

Crash statistics confirm this. The death rate in 1-3-year-old minicars in multiple-vehicle crashes during 2007 was almost twice as high as the rate in very large cars.

“Though much safer than they were a few years ago, minicars as a group do a comparatively poor job of protecting people in crashes, simply because they’re smaller and lighter,” Lund says. “In collisions with bigger vehicles, the forces acting on the smaller ones are higher, and there’s less distance from the front of a small car to the occupant compartment to ‘ride down’ the impact. These and other factors increase injury likelihood.”

Here’s how the pairs of cars fared in the Institute’s new crash tests:

Honda Accord versus Jazz: The structure of the Accord held up well in the crash test into the Fit, and all except one measure of injury likelihood recorded on the driver dummy’s head, neck, chest, and both legs were good. In contrast, a number of injury measures on the dummy in the Fit were less than good. Forces on the left lower leg and right upper leg were in the marginal range, while the measure on the right tibia was poor. These indicate a high risk of leg injury in a real-world crash of similar severity. In addition, the dummy’s head struck the steering wheel through the airbag. Intrusion into the Fit’s occupant compartment was extensive.

Mercedes C-Class versus Smart Fortwo: After striking the front of the C-Class, the Smart went airborne and turned around 450 degrees. This contributed to excessive movement of the dummy during rebound – a dramatic indication of the Smart’s poor performance but not the only one. There was extensive intrusion into the space around the dummy from head to feet. The instrument panel moved up and toward the dummy. The steering wheel was displaced upward. Multiple measures of injury likelihood, including those on the dummy’s head, were poor, as were measures on both legs.

Toyota Camry versus Yaris: There was far more intrusion into the occupant compartment of the Yaris than the Camry. The minicar’s door was largely torn away. The driver seats in both cars tipped forward, but only in the Yaris did the steering wheel move excessively. Similar contrasts characterize the measures of injury likelihood recorded on the dummies. The heads of both struck the cars’ steering wheels through the airbags, but only the head injury measure on the dummy in the Yaris rated poor. There was extensive force on the neck and right leg plus a deep gash at the right knee of the dummy in the minicar.

Keeping in mind that the report is based on American drivers, this does not mean that sales of small cars will be going down anytime soon. Economic issues dictate what car a person can afford, and while many GCC-spec small cars here don’t even come with ABS or passenger-airbags like they do in America, few people here have ever been afraid of driving these little things. In the end, the best way to avoid a crash is to be aware of your surroundings. Of course, driving something a bit larger always helps, but there seems to be more people dying here in accidents involving big Mercs and tall 4x4s instead.