Myself driving through the Clyde Tunnel in Glasgow. From Whiteinch to Linthouse. At some point, I also really need to get my Lycra on and cycle through it! Using the cycleway and a pedestrian path that runs below each of the vehicle tunnels. Bad that I have never done it, since I grow up near to Glasgow!
For more information about the Clyde Tunnel check out en.wikipedia.org/wiki/Clyde_Tunnel.