The problem is that without incentives other than gridlock to get people out of their cars, better roads simply lead more people to say "Hey, traffic isn't that bad after all" and decide that driving is the best way to get where they're going. Ten or twenty thousand people come to that same conclusion and guess what... traffic is bad again. Perpetually widening roads or building new ones is pointless, because every time you do that more people decide to drive more often or from further distances, which quickly negates any benefit of the road improvements. We might as well flush that money down the toilet.
With that in mind, things like tolls or more expensive parking start to make more sense; then we can have less gridlocked streets that stay that way. If even a fraction of the money that is invested into roads and highways every year was put into transit was added to the income from the tolls and parking fees, then the transit system would eventually become decent enough that people actually wouldn't mind taking it. Or so the theory goes...