What is the correct following distance for a tractor trailer?

The correct following distance for a tractor trailer is at least four seconds behind the vehicle in front of you. To determine the four-second following distance, choose a fixed object on the road ahead; when the rear bumper of the vehicle ahead of you passes the object, start counting: one-thousand-one, one-thousand-two, one-thousand-three, one-thousand-four. If you reach the object before you get to four, increase your distance.