There are some complicated rules for the boat tails for trailers, unless they've been recently relaxed. They are built to open and close because they're not allowed everywhere ... typically you can use them on the freeways/interstates but not in the city. I can see freezing being a bit of a problem up here but usually it's something you put on a long haul truck, so you open it when you can. At the place I was at until Oct 2015 they were experimenting with all of the different aero enhancements. Bottom line all were success stories.