I'm opening this thread to ask about signing advising drivers to maintain speed as they go through underwater tunnels. Because these generally have a pronounced sag to go underneath shipping channels, vehicles tend to hold their speed (if not speed slightly) while going downhill and then slow down going uphill, leading to congestion.
I dimly recall that the I-95 Fort McHenry Tunnel under Baltimore Harbor had VMS telling drivers to "Maintain 50 MPH," but it has been 20 years since I last used it, and I don't see any such signing for it in StreetView. I also see nothing for the I-10 George Wallace Tunnel in Mobile, which I believe is the only subaqueous tunnel I have used in the US in any recent year. Casual inspection of the I-664 Monitor-Merrimac bridge-tunnel suggests that some of the VMS on the bridge section default to displaying a message telling trucks to keep right and possibly also urging all drivers to hold their speed, but I can't tell in StreetView because a frame rate issue prevents the camera from recording the entirety of VMS messages.
Pretty sure that the I-64 and I-664 tunnels have VMS signage that normally says to maintain 55.
Although not an underwater tunnel, I remember there being a sign on the Cross Bronx Expwy that read "Upgrade Maintain Speed." It was a standard ground mounted sign somewhere in the area of Webster or Jerome Av, if I remember correctly (didn't find it when I tried to take a look now).
Here's an example of one such sign, in the CBBT: https://www.google.com/maps/@37.0382504,-76.0740559,3a,47.2y,241.68h,93.87t/data=!3m6!1e1!3m4!1sxKQ5gHoutZ3LocCD4AlC2A!2e0!7i13312!8i6656
I think this uphill-grade warning sign is used implicitly to convey a "maintain speed" message:
A7 Elbe tunnel in Hamburg, Germany (https://www.google.com/maps/@53.5419398,9.9184215,3a,15.2y,314.95h,90.11t/data=!3m6!1e1!3m4!1siCcxmeM0ijUCmdwZyXU9HQ!2e0!7i13312!8i6656)
(I checked and Germany does in fact have separate standard warning signs for downhill and uphill grades. The former has sign code 108 with the thick end of the wedge on the left, while the latter has sign code 110 with thick end on the right.)