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Its the law in some european countries, Somewhat rare in the usa.

Yes.

I'm American, so — you know. It does make logical sense to do so, but I've never known anybody who keeps a safety vest in their vehicle. Is this something like the metric system?

It is required by Law, like the warning triangle, requirements are different in different countries, some mandate also a first-aid kit and/or a fire extinguisher:

https://trip.studentnews.eu/s/4086/77076-Safety-equipment-fo...

Lots of people keep the hi-vis vest together with the triangle/spare wheel, etc. in the trunk, but in theory these high-vis vest should be put on as soon as you get off the vehicle, so a better place is the glove box or (even better if available) the pockets on the rear of fronts seats.

In Austria, if you have your hi-vis vest in the trunk, an officer will at least let you know their disappointment in case of a control. Driving schools also teach you to put it in the glove compartment.

I do.