When you tailgate other drivers (drive closely to their rear bumper):

Most rear-end collisions are caused by tailgating. To avoid tailgating, use the “Three Second Rule.” When the vehicle ahead of you passes a certain point, such as a sign, count “one-thousand-one, one-thousand-two, one-thousand-three.” If you pass the same point before you finish counting, you are following too closely.
DMV Writen Test