> We're not talking about building a basement for the entire roadway

We are talking about a basement for the entire roadway if "[m]aintenance crews can just drive through in cars and do their jobs, without stopping traffic and digging out pipes," like the GGP was talking about. Also, that's what all the pictures in the previously linked https://en.wikipedia.org/wiki/Utility_tunnel look like.

> ...we're talking about dropping modest size pipes under the sidewalks (or in many places, the lack of sidewalks) and enabling access through manholes.

I think you have a different idea, which sounds like conduit or something between conduit and a full tunnel.