The two-second rule is used to determine

A safe following distance is at least two seconds behind the vehicle you are following. This lets you see around the vehicle ahead and gives you enough distance to stop suddenly. To estimate the time, wait until the rear end of the vehicle in front of you passes any distinct and fixed point on the roadway (e.g. a road sign, mailbox, line/crack/patch in the road). However, don't take your eyes off the vehicle for more than a second or that would defeat the purpose. As you count to yourself the elapsed time in seconds, the front of your car should pass the same point no less than two seconds later. If the elapsed time is less than this, increase the distance, then repeat the method again until the time is at least two seconds.