If you shut down one freeway in the LA or SF area, every other one grinds to a halt also- including essentially all non-dead-end local roads... you might as well shut them all down at once, but there aren't enough road crews to repave them all in a week.

Above a certain population density there isn't really any way to use cars that isn't awful. When I was in Socal I would often meet people 10-20 miles away, e.g. for an after work bar trip and ride my bicycle - and beat all of the car drivers by a long wait.