The Dangers of Tailgating
You need to be at least two or three seconds behind the car in front of you, which will give you just enough time to stop or change lanes to avoid a collision if the other car suddenly brakes, gets a flat tire, or has another problem.
What to do
Pick a landmark along the road and when the car ahead of you passes it, begin counting. If you reach the landmark in less than two or three seconds, slow down and back off, says Dinh-Zarr. If you are in rain, fog, or other poor conditions, you need to be six to nine seconds behind the car in front of you. Shake a tailgater by moving to the far right lane, or consider pulling off to the side of the road so he can pass.