> The solution to this problem is utility tunnels. A tunnel network under road surface just for plumbing and cabling. Maintenance crews can just drive through in cars and do their jobs, without stopping traffic and digging out pipes. Many ultra-modern cities have one [empahsis mine].

That does not sound like a general solution to the problem, because it would be fantastically, unreasonably expensive to put one under every road. Seems like something that would only be reasonable in a 1) particularly expensive central business district of a 2) city being built from scratch.

IIRC, some of the biggest US cities don't have separate storm and sanitary sewers, because the cost of retrofitting an existing city would be prohibitively expensive. Installing utility tunnels everywhere would be even moreso.

They don't have COMPLETE and PERFECT separation of storm and sanitary sewers, but they are substantially separate systems at this time almost everywhere. They just have a finite capacity, and the overflow often ends up mixing in older cities during storms or "floods" (defined tautologically).

The cost of retrofitting an existing city aside, the sanitary sewer is a subgrade utility tunnel, by design and by cost footprint. If you're already digging a big ditch and installing infrastructure there, it doesn't cost much more to have space for other utilities. We're not talking about building a basement for the entire roadway, we're talking about dropping modest size pipes under the sidewalks (or in many places, the lack of sidewalks) and enabling access through manholes.

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.