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Status Report, Vol. 44, No. 4 | SPECIAL ISSUE: CAR SIZE, WEIGHT AND SAFETY | April 14, 2009 Subscribe

Physics dictates crash outcomes

The poor performance of all three micro and minicars in frontal impacts with midsize cars isn't surprising. It reflects 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 in velocity, the greater the forces on the people inside and the higher the risk of injury.

The second physical factor affecting injury likelihood 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, provided vehicle designers take advantage of the extra length.

These two factors, size and weight, have separate effects, but they're highly correlated. In theory the lighter weights of smaller cars could be offset by increasing the sizes of their front ends, keeping weight down by using materials like aluminum, plastic, or titanium. But this typically doesn't occur because such materials cost so much.

Characteristics including the stiffness of a vehicle's front end also influence the outcomes of crashes. However, size and weight are the basic influences.

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 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 people in the smaller, lighter vehicle are more likely to be injured.

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

"Some minicars are definitely more crashworthy than others," says David Zuby, Institute senior vice president for vehicle research. "So it pays to compare their safety ratings. But as a group minicars do a comparatively poor job of protecting people in crashes, simply because they're smaller and lighter. In collisions with bigger vehicles, the forces acting on the smaller one 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."

Fatality risk in minicars is high in single- as well as multiple-vehicle crashes. 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 rates in single-vehicle crashes of larger cars are because many objects that vehicles hit aren't solid, and big, heavy vehicles have a better chance of moving or deforming the objects they strike. This dissipates some of the energy of the impact," Zuby explains.

Insurance claims filed for injuries under personal injury protection coverage also are higher for minis than for midsize cars. Overall losses, which reflect both claim frequency and severity, are 193 for 4-door minis versus 147 for 4-door midsize cars (100 is the average for all cars).

Mini-vs.-midsize tests show size is key

A series of tests in which a microcar or minicar was crashed into a midsize vehicle shows the important role that size and weight play in vehicle safety.

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