Intheburbs, thanks so much for taking the time to hunt down that entrance and take that photo and post it -- very helpful and thought-provoking!
That warning could have been clearer, but that is pretty clear. Certainly no excuse for the bus driver's decision to enter. Steve is right, though -- "Impaired Vertical Clearance" might be better than "passenger cars only."
I can easily imagine a tired and not-very-smart and in-a-hurry person towing a trailer who says, "Well, I guess that means no commercial traffic, and I am not commercial, so here we go . . . . " That is not to say that this would be a good excuse. But safety issues sort of have to be aimed at the lowest common denominator. Merely to say that the warning is "probably adequate in most situations" is not good enough.
How much more would it have cost to provide a really clear warning, versus the cost of damage to the bridge and the resulting litigation by the bus passengers? Not to mention the risk of injury to innocent drivers who are hit by falling debris.
