Really? What makes you think that'll be the case, once the cars are talking to each other? Get rid of every stoplight in your town and see what the "physics" looks like then.

The basic capacity of a busy road is not significantly less than that of a train, and we already have thousands of roads. We just don't use them very intelligently.

Or, to put another way: a fleet of networked autonomous vehicles is a train. Put money into making that happen, and I'll vote for it. I'm definitely not voting for $50 billion choo-choo trains, or whatever 1890s-era solution the urban transit planners in Seattle are dreaming up this week.