关于大小汽车安全性比较的补充Small Car vs. Large Car Crash Safety

In a previous discussion, I tried to argue that when choosing a safe car, large vehicles are not always the best choice, and that good design is more important than vehicle size and mass for crash safety.  My opinion generated a lot of responses, so I would like to provide more explanation here.

First, I am not arguing that people should only buy small cars. Comfort, utility, style, etc. are all good reasons to choose a large car. My point is that concerns about safety should not prevent people from choosing a smaller car.

Here are two facts supporting my position:
1) Most traffic deaths in the U.S. occur in crashes involving either roll-over, collision with a rigid object, or being struck in the side by another vehicle. In these cases, greater vehicle mass does not provide additional safety.
2) Smaller vehicles are often more maneuverable (with shorter stopping distances and faster slalom speeds) and therefore can more easily avoid an accident.

The data for fatal crashes in the U.S. shows that for cars, only about 50% are collisions with another vehicle. The other 50% of fatal crashes involve a roll over, or collision with a rigid object like a tree, pole, bridge,etc. In these cases, the safety of the vehicle depends on
a) an effective restraint system (seat belts and airbags)
b) effective energy absorption in the crush areas (front end or rear end)
c) a strong cabin area that does not collapse in the crash
None of these items are automatically better in large and heavy vehicles. In fact, for item c, the roof of a heavier vehicle is more likely to be crushed in a roll-over accident than a lighter vehicle. This is one reason why rollovers in SUV's cause a higher percentage of deaths than in cars (in addition to their higher center of gravity.)



Of the 50% of crashes in cars which involve another vehicle, about 70% are either are rear-end collisions or side-impact collisions. In both of these cases, the most important factors are restraint system design (like side airbags and seat design) and cabin integrity, and not vehicle mass. To demonstrate this, the side impact results from the Insurance Institute for Highway Safety (IIHS) are shown below. (see http://www.iihs.org/ratings.)



Because these results use the same "test sled" that hits the side of the vehicle, the results are comparable between vehicles of different mass.



Because of the possible injury to the torso area, (like broken ribs, or pelvis), the results for BMW 5-Series are worse than the Toyota Yaris, even though the BMW is larger and weighs about 1200lbs more. The difference in performance is caused by differences in the design of the side torso airbags and the body structure. In the two pictures below, it is clear that the center pillar of the BMW intrudes further into the driver's seat.


 BMW 5Series


 Toyota Yaris

The rest of the fatal car crashes are caused by a head-on collision with another vehicle, and I agree than in this type of accident, it is safer to be in the larger car - the BMW and not the Yaris. But this type of accident accounts for only about 15% of all fatal accidents, and shouldn't be the only basis for determining the overall safety of a car.

I don't think that it is possible to say which will be a safer car overall - a large one that is good in head-on collisions or a well-designed small car that is very maneuverable and good in side-impact or rollover accidents. The answer depends on the driver and the driving conditions. In some rural parts of the U.S., two lane highways with no dividing barrier are common, and head-on collisions may be more likely. Also, new or inexperienced drivers are more likely to strike another vehicle, and less likely to benefit from the maneuverability of a small vehicle. A very small car might not be the best choice for them, although considering the safety of other drivers, they should also avoid a vehicle that is too large.

For me, I prefer smaller cars because they are more fun to drive, and more fuel efficient. I'm not too worried about the safety of well-designed small cars, and I don't think that you should be either.