[MilaM]

I live in a house next to a moderately busy street with car traffic and also some public transport (bus lines). I noticed that the windows (and frames) facing the street get dirty much faster than the windows facing the garden. The dirt on the street side is also pretty gross, sticky and hard to clean. It's just an anecdotal observation, but I could not come up with a better explanation so far.

[FredPret]

I had a balcony overlooking a highway in Toronto once, and it got super grimy as well. I think all the different kinds of car emissions combine into some sort of super bad tarry crap that then collects every kind of passing dust particle.