Aside from the density issue which others mention, self driving cars are un-proven technology. I get that a lot of smart people are working on them, but it's on

Aside from the density issue which others mention, self driving cars are un-proven technology. I get that a lot of smart people are working on them, but it's one thing to test a fleet of even a few hundred cars, but another entirely to have cities with thousands or hundreds of thousands of self driving cars. This is going to take longer to roll out and have the kinks sorted than you think.