Is it even an option? Where are we getting the space to put new roads along the existing roads? Many cities don't even have space for bike lanes without taking away traffic lanes, let alone literally doubling the number of roads they have.

But the answer is, we already have segregated roadways for self-driving vehicles. They're called rails, and trains drive on them. People still drive cars, surprisingly. Because if that segregated roadway doesn't go someplace that a normal road does, people will still drive their cars.