Car size and weight are crucial to protecting people in crashes. One way to see how crucial is to crash two cars that have a lot in common other than their size and weight differences. For example, crash a microcar or a minicar with good frontal crashworthiness ratings into a midsize model that earns the same ratings and was manufactured by the same automaker. What happens in the front-to-front collision says a lot about the safety consequences of vehicle size and weight.
The Institute recently crashed a Honda Fit into a Honda Accord, a Smart Fortwo into a Mercedes C-Class, and a Toyota Yaris into a Toyota Camry (these automakers have micro and minicars rated good for frontal crashworthiness, based on the Institute's 40 mph offset test into a deformable barrier). The car-to-car tests aren't about whether one minicar is more crashworthy than another. Such information is available from the comparative ratings based on the barrier tests.
The new tests of paired cars are about the physics of crashes. Reflecting Newton's laws of motion, the results confirm the lesson that bigger, heavier cars are safer. Some minicars earn higher crashworthiness ratings than others, but as a group these cars generally can't protect people in crashes as well as bigger, heavier models.
"There are good reasons people buy minicars," says David Zuby, the Institute's senior vice president for vehicle research. "For starters, they're affordable, and they use less gas. But the safety trade-offs are clear from the results of our new tests."
As in the barrier tests the Institute conducts for consumer information, each of the cars in the frontal offset crashes involving pairs of 2009 models from Daimler, Honda, and Toyota were going 40 mph. Researchers rated each car's performance from good to poor based on measured intrusion into the occupant compartment, forces recorded on the Hybrid III driver dummy, and movement of the dummy during the impact. The main difference between these tests and those conducted for consumer information is the car-to-car versus car-into-barrier configuration.
"Sometimes the whole issue of size and weight gets obscured in the quest to buy a car with good safety ratings," Zuby says. "The ratings are important, but frontal ones can be used only to compare cars that are similar in size and weight. You can compare the ratings of the Fit and Yaris, for example, and find they both earn good overall scores. But you can't compare these cars' ratings with those of midsize cars — or with the ratings of cars in any other class, for that matter, because of the effects of vehicle size and weight."
The Institute didn't choose SUVs or pickups, or even large cars, to pair with the 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.
Honda Accord versus Fit
Midsize Honda Accord: Good
The Accord came through the frontal test without significant downgrades. Measured intrusion at 8 locations in the occupant compartment was in the good range, and all except one measure of injury likelihood recorded on the driver dummy's head, neck, chest, and both legs also were good. Only the value recorded on the left foot veered from good into the acceptable range (values are based on thresholds indicating injury likelihood).
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 at 6 of 8 measured locations, warranting a marginal rating for the structure. Overall, the Fit is rated poor in this front-to-front test, despite its good crashworthiness rating based on the Institute's offset barrier test. The Accord earns good ratings for performance in both tests.
Mercedes C-Class versus Smart
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.
"The Smart is the smallest car we tested, so it's not surprising that its performance looked worse than the Fit's. Still both fall into the poor category, and it's hard to distinguish between poor and poorer," Zuby says. "In both the Smart and Fit, occupants would be subject to high injury risk in crashes with heavier cars."
In contrast, the C-Class held up well, with little to no intrusion into the occupant compartment. Nearly all measures of injury likelihood were in the good range, though the measure on the head was downgraded to acceptable because the dummy's head struck the B-pillar hard. Still, this was a good performance overall.
Toyota Camry versus Yaris
There was far more intrusion into the 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.
Like the Smart and Fit, the Yaris earns an overall rating of poor in the car-to-car test. The Camry is acceptable, which doesn't match its good rating in the Institute's 40 mph barrier test, despite the similar speed and offset configuration. Still the midsize car fared much better than the mini.
Yaris in barrier test: Good
Yaris into Camry: Poor
Barrier test vs. car to car: Car-to-car crash tests often are more demanding than the front-into-barrier tests the Institute conducts for consumer information (see ratings). A basic reason is that the barrier test mimics a frontal crash between identical cars — a Toyota Yaris into a Yaris, for example. Because the midsize Toyota Camry weighs more than the Yaris, it inflicted more force on the minicar, compared with a barrier test.
Drivers of minicars aren't likely to confine their crash experience to other minis. As the smallest cars on the road, they're far more likely to collide with bigger, heavier vehicles. This is when the safety consequences resemble those in the crash with the Camry — or worse.
Another consideration is that, while the Institute's barrier approximates the front of another car, it can't be designed to mimic the various fronts of hundreds of different cars. This helps explain why the Camry performed worse in the test with the Yaris than in the barrier impact that approximated a crash with another Camry — something about the Yaris' front end was more difficult to manage.
Laws of physics prevail
Some proponents of mini and small cars claim they're as safe as bigger, heavier cars. But the claims don't hold up. For example, there's a claim that the addition of safety features to the smallest cars in recent years reduces injury risk, and this is true as far as it goes. Airbags, advanced belts, electronic stability control, and other features are helping. The same features have been added to cars of all sizes, though, so the smallest cars still don't match bigger ones in terms of occupant protection.
Would hazards be reduced if all passenger vehicles were as small as the smallest ones? Yes, this would help in vehicle-to-vehicle crashes, but occupants of smaller cars are at increased risk in all kinds of crashes, not just collisions with heavier passenger vehicles. Almost half of all crash deaths in minicars occur in single-vehicle crashes, and these deaths wouldn't be reduced if all cars became smaller and lighter. In fact, the result would be to afford less occupant protection fleetwide in single-vehicle crashes.
Yet another claim is that minicars are easier to maneuver than big cars, so their drivers can avoid crashes in the first place. Insurance claims experience says otherwise. The frequency of claims filed for crash damage is higher for mini 4-door cars than for midsize ones.
There's no getting around the laws of the physical universe. The Institute's new crash tests confirm this — again.