I would guess the measurement is more like how much dirt and dust is involved with a car driving driving down the road. Like brake dust, tire stuff.. and then that stuff being kicked up from the road surface by other cars and so on.

And yes it's super filthy and gross. I live on a busy street (but not a main street, so not THAT busy)... and my bathroom windowsills turn black after about a month because of brake dust and other car grossness.