On Thursday, I mentioned that Americans have been driving a good deal less during the current economic downturn. Here’s another curious twist: Americans have also been driving much more cautiously in this current slump. It’s not just that the total number of traffic fatalities is down. The number of fatalities per mile has plummeted to all-time lows, as well. Why is that? Here’s a historical graph to get us started:

More recent data here. First, it’s clear there’s been a general downward trend: Traffic fatalities per mile have been declining steadily since the 1940s (although people have been driving more and more until recently, so the number of overall fatalities has bounced about). That larger safety trend has been chalked up to everything from better highway and vehicle design to campaigns that address drunk driving and seat-belt use.

What’s interesting, though, is that fatalities per mile often drop during a recession — and it’s dropped extra dramatically in this particular downturn. “This is the most profound departure from the general trend we’ve ever had,” said Darren Grant, an economist who studies traffic safety at Sam Houston State University (and who provided me with the chart above).

Grant notes that traffic-safety experts aren’t certain why people seem to drive more safely during a recession. One factor might be that people go out drinking less. But Grant offers up another theory: “It’s possible that it has something to do with how people value their time.” When the economy’s booming, the logic goes, people drive faster because they have places to be. That same principle might explain why people walk measurably faster in cities with high productivity, like, oh, Manhattan (although this is a complicated topic, see here for a fascinating discussion).

So it goes with driving. “In study after study we see that economic factors like the employment rate have a big effect on traffic fatalities,” Grant said.

Weirdly enough, the effect seems to be especially pronounced this time around — another small indication that this downturn is just plain different from previous ones.