Sometimes it's so bad that the cars can't even more, at least not without external assistance:
…heads begin to appear between the hoods and trunks.
Motivated by a meeting they wish to keep, men wade into the fray,
examining the crystalline structure of the traffic, looking for gaps,
irregularities, wiggle room. Because there’s always wiggle room. Six
inches here, a foot there, and this makes all the difference. It’s
reverse Tetris: move one this way, move another that way, and suddenly
some cars are free.
The amazing thing is this: these men don’t coordinate their actions.
They don’t formulate strategies. In fact, they probably think they’re
working against each other–as passengers in trapped cars, they care
about helping the other cars move only insofar as it helps get their
own car on its way.