This is a tough study to make.
For instance, a young driver might well be more careful with a newer car, because there is more fear of wrecking it than there would be in a beater. It is not necessarily the case that the car is inherently safer.
Also, youths might be more tempted to do stupid things in smaller, sportier cars. No one is going to weaving in a Buick Park Avenue, but last Sunday on I-85 from Atlanta, the crazy speeding weavers were driving a Dodge Neon and a Chevy Aveo.
If they’re weaving, it’s probably because slower traffic isn’t keeping right. It is a common problem.