DOT and SOLAS conspicuity tape (the red and white reflective stuff on semi trailers and so on) is mandatory for certain commercial vehicles, but also works great on bikes, scooters, strollers, backpacks (or rigid tags attached to backpacks, many varieties are kind of stiff), boots, helmets, and other things that you want not to be run over by vehicles with headlights.

It comes in a few grades, the better ranks are magnificently, astonishingly bright! You can also get more flexible, stretchy types made to be stitched or glued onto fabric.

I outfitted my classy black bike and bike gear with a bunch of material from reflectivepro.com, they're easy to deal with even for consumers who only need a little bit of material.