Motor vehicle death rates are on the decline, but according to the Insurance Institute for Highway Safety, it's not a result of drivers' increased caution.

A recent IIHS study, "Trends Over Time in the Risk of Driver Death: What if Vehicle Designs Had Not Improved?" shows that designs of passenger vehicles have been improving and becoming more occupant-protective for years. The study's main finding indicates that without these improvements, the motor vehicle death rate per registered vehicle would have stopped declining in 1994 and started increasing.

The researchers separated vehicle effects from other effects on motor vehicle death rates from 1985 to 2004 by estimating what the death rate trend would have been if people currently drove the kinds of vehicles they drove in 1985. The death rate trend given this hypothetical vehicle fleet started to go up in the 1990s, when the actual trend fell during the past decade.