Once autonomous cars become common enough, and we have a common protocol for communicating between them, I'm pretty sure the cities will be ok with it. The autonomous cars could be instructed to move out of the way for an ambulance to come through and/or stop at intersections, which would actually make things much safer for the ambulance.

Shoot, it'd be safer even if the ambulances weren't autonomous - because the cars would be reacting in a predictable manner.