It’s difficult to separate the effect of weight from the effect of size in the real world. However, a recent HLDI analysis did this to some extent by comparing hybrid vehicles with their conventional, nonhybrid twins.
These pairs are identical except for the battery packs that give the hybrids extra mass. The analysis of insurance claims found that the odds of being injured in a crash are 25 percent lower for people in hybrids than for people in the nonhybrid versions of the same vehicles. While other factors, including how, when and by whom hybrids are driven also may contribute to their advantage, HLDI concluded that the extra weight is likely a key factor.
Size also confers its own advantages, independent of mass. For example, among 2005-08 models, SUVs weighing 3,001-3,500 pounds had 39 driver deaths per million registered vehicles, compared with 51 for cars in the same weight range.
The advantage for SUVs may be connected to their taller profile. For one thing, the taller a vehicle is, the less likely it is to underride another vehicle in a crash. In addition, crash testing suggests that sitting higher makes occupants less likely to have head and chest injuries when their vehicles are struck in the side by shorter vehicles because the point of impact is lower.
This SUV advantage is a relatively new phenomenon. In the past, these vehicles had some of the highest death rates because they were prone to rollover crashes, and that tendency outweighed any advantage from their bigger size. That’s no longer a factor because of less top-heavy designs and the increasing prevalence of electronic stability control, which prevents rollovers and is now required on all new vehicles.