Stumbled across this today when I was browsing illustrations inspired by 'Hylas and the Nymphs'.
'Meet Scott Wade – a talented artist, who like every creative and curious person can’t resist a dirty rear car window. We suspect that Scott started off with clever sayings, like, “wash me”, however his later works became much more sophisticated.
He lives on a mile and a half of dirt road – caliche, as the locals call it, road-base: a blend of limestone and gravel clay. Driving over this surface results in a fine, white dust that billows up behind any vehicle driven faster than a galloping turtle, coating the rear window. Being an experienced artist it wasn’t long before Scott started experimenting with techniques to achieve these amazingly detailed and shaded drawings.'
More examples can be found on his website: http://www.dirtycarart.com/