The Peltzman Effect

Sam Peltzman taught us that wearing a seatbelt induces people to drive less safely--a study discussed in the first chapter of my favorite economics textbook. Now the Sydney Morning Herald tells us that driving a large 4WD vehicle has a similar effect:

4WD drivers were almost four times more likely than car drivers to be using a mobile phone, ....[and] using a mobile phone while driving is associated with a fourfold increase in the risk of having an accident.....

"Although 4WD vehicles are safer in a crash, their owners may be placing themselves and other road users at increased risk of injury," she said. "They take the risk because they are higher up, they feel they can see better... but the person in a car or the pedestrian on the road has a much worse outcome."

Here is the study. Thanks to John Chilton for the pointer (via Newmark's Door).

Bonus: Mistaken inference

The Sydney Morning Herald reports the following inference from the two facts given above:

That means drivers of four-wheel-drives are 16 times more likely to have an accident than other drivers.

This conclusion, however, does not follow from the premises. Here is a numerical example.

Suppose that 1o percent of car drivers use cell phones and that 1 percent of car drivers not using cell phones have accidents. Then out of 100 car drivers, we get 1.3 accidents (0.9 accidents from the 90 cell non-users and 0.4 from the 10 cell users).

Now consider 100 4WD drivers. We have 60 drivers not using cell phones and 40 drivers using cell phones. The first group generates 0.6 accidents and the second group generates 1.6 accidents, for a total of 2.2.

So, in this numerical example, the total number of accidents per 100 drivers rises from 1.3 to 2.2, which is not a factor of 16.