If you have more car-lengths of empty lane in front of you than you do cars backed up behind you, you're impeding the flow of traffic.
Bad rule. You're actually smoothing traffic by creating anti-traffic in front of you to absorb minor slowdowns. Drivers who keep distance in front of them are essential to dissolving traffic jams. See http://trafficwaves.org/trafexp.html for more details.
Thanks for that link - besides potential mass transit solutions, waves are all I think about when stuck in traffic. One thing to note, though, the linked page is wrong on the point that outflow can't be changed from a jam - if everyone in the jam accelerated as quickly as possible with almost no lag between each other, the entire jam point could move and eventually disperse. This would probably take coordination from networked robot drivers to pull off, though :-) since one slow driver could ruin the whole thing.
1. If you have more car-lengths of empty lane in front of you than you do cars backed up behind you, you're impeding the flow of traffic.
2. If you have no room in front of you and some asshole is tailgating you, he's an asshole.
Exceptions apply, of course — they're only rules of thumb, after all — but they've served me well enough.