Err, they're not going through the tunnel. They're too tall for it. They crash into the roof of the tunnel, causing severe damage to the infrastructure, their trucks and anyone who happens to pass by.
There's likely some space left between actual tunnel height and truck height limitation (for safety), and clever driver's incentive to go through the tunnel (if it's a nice shortcut) might be stronger than any kind of sign.
Can you, maybe, also cite a trustworthy source on how many trucks actually went through the tunnel without a problem? I imagine there're clever drivers who know precisely metrics of their vehicle and tunnel regulations. I think we rarely hear about those cases on the news, though.
Edit: if you're responding “No” to whether there's leeway between actual tunnel height and regulations, I'd like to hear a source on that as well—I couldn't find one, but common sense suggests that in such a case then there'd be enough incidents of trucks with allowed height damaged because of, say, a bump on the road.
