[DanBC]

I'd leave at least a minimum safe distance between my vehicle and the one in front.

But Parent suggested that it's a good idea to leave more space in case someone jumps in. I'm not sure I understand that - if they jump in it's always dangerous because you don't know what the person behind you is going to do and you don't know when the jumper is going to jump in. Thus, you want to try to prevent them jumping. This is achieved not by giving them more room (which encourages jumping) but by leaving less gap (reducing jumping) but still leaving the safe minimum.

[peterwwillis]

The stopping distance of my car (a Lexus IS300) at 70mph is 171 feet. So I know I need over 14 car lengths in front of me for me not to hit anyone with brakes at full power.

Do I actually leave that much space in front of me every time I drive 70mph? Hell no. But the slower the cars beside me are going, the quicker i'm going to hit one if it pulls in front of me. If they were going 70mph too, I would only have to brake down to, say, 65mph, which would should only take a second. But if they're going 10mph, I now have to come to an almost complete stop to prevent from hitting them, which as you can see above is a lot of car lengths.

[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.

[gknoy]

I prefer to be pessimistic about other drivers' ability to judge whether they have enough room to cut in front of me. I'd rather not try to go fast enough to "discourage", as there will inevitably be some person who either doesn't notice me, or misjudges my speed (via a side view mirror) and mistakenly thinks he can cut in front of me safely.

A lower speed increases the chances of this, but reduces the risk of a catastrophic result. ;)