[crzwdjk]

Unfortunately, it doesn't always work out that way, because congestion tends to be the limiting factor for demand for car trips thanks to a very low perceived marginal cost. So as soon as congestion reduces somewhat, that extra capacity suddenly makes it more attractive for the marginal driver to take a marginal trip, and traffic increases (but not quite all the way back up to its original level).

[carlmr]

Not if more people forego the car entirely. The marginal trip costs are high if you don't have a car. Even if you could rent one cheaply over the weekend, it's more marginal cost than if you have it in your garage.

A lot of European cities with good public transport and narrow medieval roads still have less traffic than American cities with 4 lane roads.