Just an opinion, if there were “frequent rock falls” I would think the tunnel would be caved in?
I really didn’t feel concerned, we were actually stopped by a large quantity of water of unknown depth, without a lifeguard.
The “frequent rockfalls” refrain might be an attempt to keep people out for safety… if you scare them enough, the sheeple sometimes obaaaa you.
Actually in spaces like tunnels, much like abandoned mine shafts, build ups of toxic gases are much more dangerous and prevalent than rock falls, if not toxic then just displaceing normal oxygen…
I looked back at my notes and it was Wrays Hill, the one that has several cave-ins that Chris was talking about. But there is the famous Broad Top Snake to look out for 
Interesting that with Google Satellite View you can see the cave-in on Wrays Hill. Looks like a navel in the middle of a farm field! I couldn’t find the Sideling Hill tunnel on Google Maps. The roadbed in that area is to obscured for me to locate it. The big mule-shoe curve at Coles shows up real good though.
Linkish?