1) You don't really need drivable tunnels, just tunnels big enough to get to the stuff without digging it up.

2) Don't retrofit. Rather, if you dig up a street you put in the tunnel while you're doing it. Eventually all the important roads end up with tunnels.

Or you can just run through a city core from one side to the other side like subways. It doesn't have to strictly follow topside road networks, it's just that roads are easy target for permitting purposes.