This reminds me of an issue I encountered while on vacation recently at my mom's house in Massena, NY (Canadian border, St. Lawrence River, north of Syracuse and Watertown). In the state park area by the Seaway (a series of locks and dams, including a large hydroelectric dam), I was glad to see a lot of "Shared Roadway" signs with bicycle icons. However, at one point there is a short tunnel underneath the shipping channel that sounds basically like the DC tunnels the OP describes. (It's pretty cool, really - where else can you go through a tunnel directly underneath a large laker navigating the lock above you?!) But at the entrances to the tunnel are signs reading "
Bicycles Keep Extreme Right".
Now, the lanes in the tunnel are narrow, and terminated at the edge with a curb. There is also a narrow sidewalk, but the sidewalk is slightly elevated with steps, making it even more useless to bikes! So my argument with the signs is that they promote the idea that cars should be able to pass bikes in the tunnel, whereas to do so is actually very unsafe!
My proposed solution, which I will be communicating to a NY state bike/ped employee in the near future, would be to replace those signs with ones reading something like "
Yield to Bicycles in Tunnel". I wonder if this would be a worthwhile direction to pursue with these DC underpasses as well? (Although I'm pessimistic, since the point of building the underpass was no doubt to streamline the flow of through automobile traffic, so allowing and even encouraging bikes would probably be interpreted as contrary to that aim.)
I think the current signage situation the OP describes is very dangerous in that it implies to the motorists that bikes have no right to be there, making the motorists even less likely to watch for cyclists, and then less tolerant of the ones they do see. The situation also of course opens the cyclist to the risk of being ticketed and fined!
Good luck, DCCommuter, and be careful!