[Kluny]

In every case, if you can't predict what people around you are going to do, it's better to leave more space in front. If the guy behind you is following closely and might rear-end you, leave more space so that you can slow down gradually, instead of slamming the brakes and inviting collision. If the car ahead of you might have to slam on the brakes, leave enough space that you can slow down gradually.

If someone in the lane beside you might cut in front of you, leave more space so that you can see them making their move and react in time. That reaction might be to speed up and prevent them before they change lanes, or to slow down and let them in, but in either case, leaving more space is a better idea than tailgating.