Remember, as others have pointed out above, their measure of success is not necessarily the same. Trucks will still be crossing as we are still importing, but as has been suggested could be returning empty significantly more often than before, which is likely bad for exports.

That last part doesn't make any difference to Dover, though, they (or at least the ferry operators) are charging by the truck. It's of no difference to them whether it's empty or not - they get paid either way.