Chowder,
I sometimes tow mine with the top up BUT, I'm only going from Myers to Long point and
not going 55 or 60 mph usually don't go over 45. I don't think the support poles would take the draft of a 18 wheeler going in the opposite direction at 60 or 70 when your doing about the same. It probably would depend on the height of your top and how much air would get inside.
Splitshot115