Doesn't even have to be a major metro area. Start small. Someplace without snow (Google cars still have problems with snow). Puerto Rico. The Bahamas. Hawaii. These are all small places where you can shake out the major issues before expanding.

Also, other places to start would be Nevada (as Google is doing), Texas, and other large states where there is a benefit to having something automated do the driving for 3-6 hours across large stretches of highway.

You're never going to have a major metro just ban human-driven cars overnight; you win by attrition, small bites very quickly.