Normally it is just a lever that you flick over. I know I should do it but I never bother when I am there. You can also get stickers at the tunnel to put on. Just be careful on the speed as they are hotting up over there. I used to regularly go up to 150 in the good old days but now I keep to 80 / 85 mph. I think their flash cameras can now send the bill here and it is illegal to even have speed camera listings on your tom tom type satnav.