Seems like the liability for damages caused needs to be replicated between the drivers and their hiring company, making it not worth ignoring signage like this
Not everything can be solved with economic incentives. Do you really think people are making a conscious decision, "I'm not paying for it, might as well peel the top off my truck and grind my day/week to a halt because my employer is liable for damages"? If that was the case, they can just drive through the water wall too. This is an attention management issue -- does the driver realize they are too tall for the tunnel. Most will be caught with the initial flashing signs, but even those high-salience signs are somewhat 'normal' and thus possible to subconsciously ignore. The water wall is very out of the ordinary so it has a maximum likelihood of capturing attention.