Originally Posted by
cyccommute
I'm even dubious about the water vapor from exhaust causing the problem. Assuming 2,2,4-trimethylpentane (isooctane), a gallon of gasoline has a mass of 2.7 kg. Combusted, it makes 3.8kg of water and 8 kg of carbon dioxide. Assuming 26 miles per gallon, .14 kg of water per mile of driving (sorry about mixing units but it's easier to think that way). For every foot of that mile, that car is putting out 0.027g of water. That's not a lot of water per foot. A cubic foot of air can carry 1.2g of water per cubic meter at -18°C. That's the exhaust of 44 vehicles assuming that the water stays in one place which it wouldn't. It's going to be mixed with a lot of dry air from everywhere around the car. The water may come out of the air but I doubt that the exhaust from vehicles is the major source.
It's typically formed at intersections where multiple cars are stopped waiting for lights and this occurs many times over the course of a rush hour. The tail pipes of lots of vehicles direct the exhaust right down to the street. Again, these cars are stopped so the exhaust is not getting disbursed over a large area as it would if the cars were moving.