frontal area requirement?
As far as I know, there are no frontal area laws. The TRUCK MANUFACTURER is the one who specifies the limit. So if the truck is 'rated' for 50 sq ft and you tow 52.7 sq ft, the police won't care one bit. However, if your truck breaks down, the manufacturer may decline to perform warrenty service, and if you were to get into an accident, a really sleazy lawyer (as opposed to a normally sleazy lawyer
) may try to make a big deal about it.
I'm not sure, since I just recently heard about 'frontal area limitations', but I suspect the reason behind this is wind resistance. If you have a large frontal area, it will be harder to pull, putting more strain on the engine and transmission, as well as the hitch and mounting. Thus, it seems likely that only the vertical component of a slanted area would count.
Let us say there are 2 trailers, both 8 feet wide and 10 feet high, with the floor 2 feet above the ground. One is straight up and down, and the other has the top 4 feet slanting backwards. Figuring the frontal area of the first one is easy, 64 square feet. I think the second one would also be 64 square feet, despite having more surface area (slanted area would be 5 feet high rather than 4 feet high (hypotenuse is square root of sum of the squares of the sides).