I'm trying to find an off route cycle route to work and the best way is through the Clyde Tunnel then via the Clyde Walkway to the city centre. It took me around 10 mins to actually find the entrance and it is not very well sign posted if at all. There is also only one cycle tunnel open and for both way traffic so keep your eye out as you can pick up some speed on the way down.
Entrance is via a call button which you press and a double gate system opens to let you through. I seemed to get locked between the two gates and had to wait 5 mins till someone figured out I was there (and pressing the button). You also have to press the button to exit the tunnel as well. It's a little creepy but much safer than it was when I was a kid.
The entrance is behind Lidi next to the Southern General Hospital and I've added a wee map below.
Click on the image to enlarge