Have a law, define a target weight + speed and then make it REALLY expensive to insure or kill people with your car.
Also in the law: if you drive around without insurance, the car is instantly taken away from you, as it is a weapon to conduct a crime :-)
Still people will drive big cars around, but the market will limit the number of people who can pay for it. And of course: new cars only and when ownership is transferred. No additional tax for existing owners.
The market would solve this problem VERY VERY quickly.