If you mean other people not in cars then picking a lighter car wont matter for the people outside. Airbags on the outside of the car are the only real solution

If you mean other people not in cars then picking a lighter car wont matter for the people outside. Airbags on the outside of the car are the only real solution. 0.5mv^2 scales too fast for m1 to matter if m1 >> m2.