That looks like it WAS a brake pad. There's no pad left on that thing. If it was being used there was not much in the way of stopping power left. It looks like only a metal backing to the pad.

 

Ouch. That person is very lucky.

 

Looks like a truck brake pad that was lost from a scrap metal run. A mufler shop or brake shop has to take their scap to the metal recycler and this could be one that fell off then got kicked up from the road by another vehicle. Regardless, this driver was lucky it wasn't higher.