Roads are filthy. Cody's Lab once did a video where he collected road dust from the side of a highway, and refined out all the precious metals from it. That's from one small section of highway, just imagine the scale of the pollution across the whole of the states. Concrete dust, rubber, metals, fuel, oil. Cars and car infrastructure is dirty business.

https://www.youtube.com/watch?v=v5GPWJPLcHg

If one are outside near any mountainous road here in Northern California, motor vehicles pass by with the odor of brake pad + brake disc (mostly brake pad?) near the bottom with a very strong, distinctive bitter sweet metallic odor and that's probably a tiny proportion of the material.