I leave enough room for me to have time to react and give the person behind me enough time to react to my breaking. Because most people drive way too close to avoid a rear-end collision if I have to slam my breaks for an emergency. Sometimes that means I'm 4, 5 or 6 car lengths away from the car in front of me, but that has the added benefit of pissing off tailgators who will almost immediately choose to pass me which works out great for me. The less space the person behind gives, the more space I leave between me and the person in front.