For the moaners who complain about bikers not paying tolls, you try and stop, taking off your gloves, digging around in small zip pockets for change and then putting your gloves back on before moving off, there would be bikes backed right through to Liverpool!!!
It's more about practicalities than anything else.
For the person who started this thread, it just so happens that you should approach at the speed it recommends on the sign, and if you do that, then there's no problem. For people not used to the area, then i would certainly recommend getting in touch with the tunnels and mentioning as a biker, the humps are not visible enough, then they might paint them bright or put up a sign warning of the height, might only take an email but for the time it's taken to comment in here, you might actually get them to make a change.
I emailed about a problem with a road by ours once, and it was sorted!! Has to be worth a try. Good luck.