Three 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.
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.
“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.”
The death rate per million 1-3-year-old minis in single-vehicle crashes during 2007 was 35 compared with 11 per million for very large cars. Even in midsize cars, the death rate in single-vehicle crashes was 17 percent lower than in minicars. The lower death rate is because many objects that vehicles hit aren’t solid, and vehicles that are big and heavy have a better chance of moving or deforming the objects they strike. This dissipates some of the energy of the impact.
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.
In response, the Obama administration announced it is boosting the fuel economy standard for cars, beginning with 2011 models, and instituting a size-based system to set fuel economy targets like the one already in effect for SUVs, pickups, and vans. This system will mandate lower fuel consumption as cars get smaller and lighter, thus removing the incentive for automakers to downsize their lightest vehicles to comply. It also could mean that technology currently used to enhance horsepower would go instead to reduce gas consumption — a direct safety benefit because less powerful cars have lower crash rates.
“Fifty-five was adopted to save fuel, but it turned out to be one of the most dramatic safety successes in motor vehicle history,” Lund concludes. “The political will to reinstate it probably is lacking, but if policymakers want a win-win approach, lowering the speed limit is it. It saves fuel and lives at the same time.”