For the first time more women in the U.S. have a driver's license than men. Its just one of the major shifts in modern driving.
According to the same University of Michigan study, the proportion of teens and young adults with license is down, especially for males. Men 25-29 showed a big 10.6% drop in license holdership in the 15 years up to 2010.
That leaves a greater share of women out there driving, and they tend to buy smaller cars that use less gas and they tend to have lower fatality rates on the road.
Another part of what's going on here is rising use of the internet and mobile devices as vehicles of freedom, contact and expression instead of cars among some teens and young adults.
And cars aren't quite as much of a hobby in the 21st century, now riddled with so much digital electronics that shop class has become an anachronism rather than the teenage rite that leads to a lifelong interest.