Some believe that smaller, more fuel-efficient vehicles are a key component of any sensible climate policy. But smaller cars, all else equal, tend to be less safe cars. Indeed, a recent report on much ballyhooed "minicars" finds that they are less crashworthy than normal sized vehicles.

The Insurance Institute for Highway Safety (IIHS) reported that in a series of test crashes between minicars and midsize models, minis such as the Smart car provided significantly less protection for their passengers.

The tests did not involve the much ballyhooed mismatches between subcompacts and Hummers, but measured the effect of relatively modest differences in size and weight. Even though the Smart car and other minis such as the Honda Fit and the Toyota Yaris have fared relatively well in single-car crash tests, they performed poorly in these two-car frontal offset collisions. In the words of IIHS president Adrian Lund, "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."

That difference is reflected in the real world. The death rate in minis in multi-vehicle crashes is almost twice as high as that of large cars. And in single-vehicle crashes, where there's no oversized second vehicle to blame, the difference is even greater: Passengers in minis suffered three times as many deaths as in large cars.

Just as alternative energy sources are not free of environmental impacts, more fuel efficient vehicles have their downsides as well.