My friend had Left 4 Dead 2 before me, and he said this achievement to me and at first i thought, 'Thats impossible'. But when i played, i only noticed that there is only a few cars. But this achievement is so much easier than it looks. All you need to do is, grab a melee weapon (there should be one right behind you as you climb up into the area) and only jump on the white cars, do not go anywhere near the red ones, plus, make sure there are no smokers around, because if there are, they make drag you into an alarmed vehicle. Just make sure you use a melee weapon, because if you use a normal gun theres a slight chance you may shoot the alarmed cars. As soon as you walk into the safe room, the achievement should pop up.