[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.