Well, here in OK they have those yellow becons/strobes on top of school busses, that haul people/kids. Maybe those trucks are hauling ????? in the containers? (Autsch, not a very nice comment, since I am "one" ("????") too, but the little devil on my shoulder made me do it....)
Lights or not, I hope that they do not haul people..too many die that way, horrible way to go, no one deserves it, regardless.