We have to carry them by law while driving in the EU-- they are required to be reachable from the drivers seat. We are required to wear them when leaving a disable vehicle at night. Of course, we don't have shoulders that are wider than the lanes of traffic, like in the US.
pretty much all bicycle commuters wear some sort of reflective vests when riding in low light--- and everyone uses lights. I don't understand why reflective vests are not more common in the US