Traffic

Driving isn't a solitary thing, it's a group activity.  You can read the mood of someone from their driving, who's impatient, who's placid, who should be avoided because they drive up your exhaust pipe.  The trouble is, you don't often like the group.  If there's a lot of traffic around, it's like cramming twenty teething toddlers into a small bathroom and locking the door.  Statistically, there's about 1/6 of the driving population who are radically more aggressive behind the wheel, the rest of us are consistent in and out of the car (New Scientist statistics from a while ago).  Those are the scary ones.  Traffic flow is one of those turbulent things that really irritate people trying to come up with a decent predictive model.  The one thing I do recall from the studies is that when everyone is doing the same speed, it's an unstable state.  You will either go back to normal, multi-speed flow, or create a tailback.  All it takes is for a couple of people to hit the brakes at the wrong moment, and you're in a jam.

Yes, I am driving home on two high traffic roads at peak time, and have to pass the junction where a third road stops dead and dumps everyone out at once.  Lousy design, the road should have been extended, but some bright spark built a Target store at the end of the road.