It's not very useful for transit now. The tunnels were built so that the only vehicle access to them would be from Connecticut Avenue. This presents two problems:
- Metro goes under Connecticut, so it handles most of that traffic. There are some bus routes on Connecticut, including 1 with pretty high ridership, but they're mostly used for neighborhood-shuttle type trips that are so short that it would be inconvenient for riders to have to go into a subway.
- The other bus routes that come in or near Dupont Circle use other streets. Most notably Massachusetts Avenue and P Street. It might be worth it to hub all the buses from Mass, P, and Connecticut into a single station in order to facilitate transfers and efficiently provide services, but doing so would require a bunch of new tunneling to provide exits off Mass and P into the loop. Not only would that be very expensive, it would also make Mass and P more like highways, which would be such a bad side effect that it would more than negate any benefit.
So yeah. Using it for buses isn't on the table.
There was a proposal to use part of it as a bike station, which I think is a good idea, but that would only use a tiny portion of the square footage.
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