Any amount of rubber would only work up to a certain speed. And all modern cars are designed to provide 100% safety to passangers up to 30mph - crumple zones, multiple airbags, engine block sliding underneath the body of the vehicle, early collision warning, automatic breaking and so on. Encasing cars in rubber would not improve anything above 30mph, while I am fairly certain would increase the weight of the vehicle - making it consume more fuel and also heavier vehicles take longer to stop,so the benefit might actually be null.
