You will not find specific guidance regarding hazmat and tunnels in the FMCSR's because the state and local governments determine which tunnels cannot have hazmat hauled through them.
I pull a hazmat tanker....and general rule of thumb is: If you have a placarded load....tunnels are a
NO GO!!
Now, my loads are 45,000 lbs of liquid in an unbaffled tank, so in some circumstances, the rules that affect me are different than those that affect a van or reefer driver pulling hazmat.
For instance: I cannot take a placarded load of certain classes of chemicals through the tunnels on the PA Turnpike, however I did once pull a load of class 5.1 Oxidizer from Philadelphia all the way across PA to Ohio through the tunnels,
and I did it LEGALLY!!
The trick is that you have to call the PA State Police and ask for the Hazmat Routing Office and discuss it with them. In the case of the load that I was hauling in a van.... it was bags and their weight and packaging met the required conditions to transport the load through the tunnels. The PA Officer even gave me his name and badge number in the event I got stopped.
On the other hand....if I tried to take that very same load south on 95 and made the tragic mistake of going through "the whole in the wall" which is the tunnel on I-95 in Baltimore.... I would probably still be in prison and working at least two jobs to pay off the fines and wouldn't have a CDL anymore.
Hazmat routing is not a joke. In this post 9-11 world, you can bet your bottom dollar being in the wrong place with a hazmat load is a very bad thing with dire consequences to your wallet, freedom and future.
When in doubt....call the State DOT or State Police in the states you will be traveling through... if they give you the "go", just get a name and badge number and you're good to go. If they say "no".....they mean no.
In the from of the Motor Carriers Atlas there are all the numbers listed for the different state agencies that you might need to talk to.