Most of the highway trucks here use 200', though I've yet to see anyone take heed. They are full width and reflective orange on the back of spreaders. Might be easiest to have them made up, but I do remember seeing them in a highway safety catalog somewhere. I'll poke around my catalog collection and try to find them.
I have a reflective orange KEEP BACK 50' sign on the back of my spreader but I have to wipe it off after each lot as it gets covered in salt dust I'll take a pic of it sometime today & before & after salting
I got mine out of a safety catalog last yr., I'll look up the invioce as to where & how much It was aluminum & I figure it's just another way to cover my a$$ if I would end up in court as to show I tried to warn the public to keep out the way.