ANN ARBOR—Rising fuel prices, resulting in less driving, may very well be a reason for the decline in traffic deaths, as recent reports have suggested. But a new report by the University of Michigan shows that something more may be at play?a major shift in driving behavior.

According to Michael Sivak, research professor and head of the Human Factors Division at the U-M Transportation Research Institute, changes in gasoline sales and miles driven cannot fully explain the reduction in motor vehicle fatalities.

While the decline in traffic deaths has outpaced the drop in gas sales and number of miles driven since at least last year, the change has been especially noticeable since this spring.

Motor vehicle deaths plummeted 22 percent in March and 18 percent in April, while gas sales decreased about 3 percent and 1 percent and estimated miles driven fell roughly 4 percent and 2 percent for each of those months. The data are based on year-to-year percentage changes and are not available yet for May and June.

“Should the March and April trends continue, the 2008 annual fatalities would drop to under 40,000 for the first time since 1961,” Sivak said.

There are several possible explanations, he says, for the fact that recent decreases in motor vehicle fatalities are substantially greater than decreases in gasoline sales or estimated miles driven.

First, the reduction in distance driven, albeit smaller than the drop in traffic deaths, might have been disproportionately greater for more risky driving conditions. For example, the reduction in miles driven on rural roads?the more risky roads?for March and April was greater than the reduction on urban roads (-4 percent vs. -2.6 percent).

Second, because of the increasing cost of gas, the amount of driving might have decreased disproportionately for people with less income. In turn, people with less income (for example, teenagers and the elderly) tend to have higher crash rates.

Third, motorists may be driving slower to save gas. It could be that the safety benefits of driving slower are proportionally greater than the fuel-economy benefits, Sivak says. The disproportional reduction of driving on rural roads—where speeds are generally faster than on urban roads—also tends to decrease the overall speed profile, he says.

“It appears that the increased cost of gasoline has finally begun to influence driver behavior both in terms of the amount and type of driving,” Sivak said. “Consequently, one of the indirect benefits of the increased cost of gasoline—in addition to, for example, a reduction in emitted CO2—is likely to be a decrease in motor vehicle fatalities.”