You are driving on the freeway. The vehicle in front of you is a large truck. You should drive:

When you follow so closely behind a truck that you cannot see the truck driver’s side view mirrors, the trucker cannot see you and has no way of knowing you are there. Tailgating a truck, or any vehicle, is dangerous because you take away your own cushion of safety if the vehicle in front of you stops quickly.
DMV Writen Test