I know people that do water ski racing and every year there is a race in Cardiff. The F1 teams with their 3.5 metric ton ski race boats (supercharged 8.9 litre 1000hp V8 big block chevy, 500+ litres of fuel etc) on 1 ton twin axels trailers towed behind coach sized campers will pay £5.40 at the toll. Then, as has been said, an empty 106 panel van or whatever behind them will pay twice as much. Crazy!!!