A dramatic decline in highway fatalities in the U.S. since 2005 is a piece of good news that’s also a bit of a mystery.

Is it the result of better vehicle safety technology? Less stupid, reckless behavior? Smarter strategies for easing teens into the responsibilities of driving? Or just an unexpected positive side effect of a slumping economy?

A new study by two University of Michigan researchers — as explored in this Wall Street Journal article — of detailed federal crash statistics from 2005 to 2008 suggests all these reasons could be behind the reduced death toll.