The answer to that is carefully. Once on the roof, good scaffolding with toe boards provides protection from anything that might drop off the roof - I went through several scaffolders until I found one who installs it the way we needed it. The ladder is always the most risky element, but can't be eliminated, but we used to get a half platform on the scaffold, two short ladders is a lot safer than one large one, and much easier for lifting tools, and removing any debris from the roof. The main materials, ie rails and panels, everyone was involved with lifting, so everyone knew exactly where everyone else is, so Hi Viz would make no difference to that, hard hat, debatable as no one was stupid enough to stand directly under gear being lifted.Are you going to answer my question?
Brings me to another true story. Commercial PV installation where roof access internally was too small for panels, and as the roof had a low wall around the edge, we couldn't use a telehandler. So plan was to hire a scissor lift as we had a nice expanse of flat tarmac right next to the building, only to be told by site H&S bod that we can't do that as scissor lifts are not designed to lift materials, OK, so people and tools don't weigh anything?