[dmvaldman]

An interesting point to mention is that the traffic "jam" moves backwards. This is because at the front of the jam, cars are able to accelerate out of it, while at the back o the jam cars are piling up into it. Seen from above the jam is a shock wave moving backwards.

This often upsets people when they get out of a jam only to see there was nothing causing it! In reality what caused it may have been much further up the road.

[AlfaWolph]

It's usually high density traffic and someone pulling a move like a forced merge which causes the person now behind them to slam on their brakes. I've seen this hyperactive "weaving" happen too many times to have it be explained only by "slow human reactions times".

[cmdrfred]

I heard a few years back (no source) that when they model traffic they can design systems that work perfectly, its human error (not hitting the gas fast enough after a green light, changing lanes to often, 18 wheelers getting in the far left lane to turn right) that make the models break down.