Best way to do this is to turn Touch Drive OFF
. This lets you steer the car instead of the game steering for you. If you leave it on, the game tries to keep you more central and away from parked cars, which you need for this achievement. It is possible to hit some with it on, but it's much more difficult.
Turn it off and now aim for parked cars/buses. You will see a pop-up on the right side that says 'Traffic down' if you've done it right. The game does track these as in-game missions, so you can see how many you have and still need.
Police cars and opponent cars do not count, but count towards