This achievement will be extremely easy to achieve if you drive like me! Don't try for this one immediately, you'll probably unlock it through natural progress like others. Anything that can be hit, AI cars, sign posts etc - Hit them all. Some items are undestructible however, so be aware of them and don't keep hitting them as they will not count! Like hitting walls, some hedges and bushes count as solid objects and won't count towards the 100.
As I said, hit cars, sign posts, lamposts and traffic lights all count.