Recent official tests confirm that size and weight affect injury likelihood in all kinds of auto crashes, and these test results may aid insurersÂ’ risk management efforts. Three front-to-front crash tests from the Insurance Institute for Highway Safety, 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.

The 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. 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.

"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."

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."

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