I’ve seen groups of runners wearing these things twice, and it defies my ability to tell google what they are so I can learn more about them.
I would guess that they’re made of something like silicon, and they’re sort of a lit up backpack. It reminds me of an octopus in shape, if the octopus only had four arms. The light radiates through the ovalish main body, and through all four arms (which have to be straps as they wind around the runner’s bodies, over the shoulder and under the arm). The lights, if I correctly recall, come in at least blue, pink/purple, and maybe a yellow or green.
Whatever these lights rigs are, they do an excellent job making the runners noticeable to drivers.