Good question. I recall reading a paper some time ago that the emissions from vehicles tends to settle inland from the road due to the high temps of the exhaust raising when they come out of the engine. Luckily Tetraethyllead has been banned here for boosting octane in passenger vehicles for a long time. (We now use food to boost our octane in the USA!) No idea how long tetraethyl lead stays in the environment or the lead that leaches out of all the paint people used for 100 years on their houses.
The primary pollutants from modern spark engines are oxides of nitrogen (N2 in air getting oxidized when combustion is too hot), carbon monoxide (combustion too cool), and unburned hydrocarbons (from quench zones). While bad, these things are gasses and I’m not sure they ever settle down on anything. The real nasty stuff is the exhaust from diesel engines. You get all kinds of carbon chain compounds from a diesel engine with many of the particulates being known carcinogens. http://en.wikipedia.org/wiki/Diesel_exhaust