I'm looking for the proper word for these things you wear to avoid being driven over by cars at night.
Looks like a reflector, or more precisely, a safety reflector.
Yeah, I know, kind of obvious.
Can't tell from photo ..
This is likely a "Cat's Eye' reflector, which has distinctive 90-degree angle (from each other) dual reflectors embedded.
This sends a strong light beam directly back towards any focused light source (eg. an approaching car's headlamps).