From a truckers point of view, it's not an easy task to climb atop a trailer that's 13'6" tall after snow & ice mounded on it while u were sleeping the nite b4. No guardrails around a slick trailer top & a possible fall could be a death sentence or broken back or limbs in an instant. There's no places to clean it off safely even as a pay for it commercial biz that I know of & I been trucking for 23 years all over the states & Canada.
I've always cleaned any rocks off top edges of trailers I hauled b4 leaving but know some don't. Dumps are usually worst at that because of the nature of loading them & some not tarping or sweeping top edges after loading. The windshields from rocks I've replaced in both my big trk & personal vehicles were always related to me following too close to the vehicle ahead of me. 3 total windshield from that in 28 total years of driving. When I decided if I would adjust my following distance, I quit having broke windshields. I drive an average of 125,000 miles per year the last 20 years. Just my 2 cents from the other side of that equation.