Originally Posted by
cyccommute
The water only has to be partially open over water to have air saturated with water vapor. Rivers are usually warmer than the surrounding air as well...they have to be because water doesn't flow when it is frozen.
As to the exhaust, I would put the number of tailpipes that direct car exhaust downward in the minority to the number that direct it sideways or on a flat trajectory, not that it matters much. The exhaust is going to rise up out of the pipe very rapidly...much faster than the small amount of water in a given volume of exhaust could freeze on the ground.
Show me a credible source that's not "some guy on the internet"...I can find one...and I might believe you. Until then, yes, this is an urban legend. It has all the hallmarks..."everyone knows this", "there are lots of sources that say this", etc.
As it stands now, there just isn't enough water in car exhaust nor a high enough concentration of cars in any given area for me to believe it. There are also alternative explanations that make far more sense You might as well say that human breath is responsible for ice on sidewalks.
Lots of bridges ice up without being over water at all, -overpasses for example.
Whether the exhaust is directed down or not, a lot of it comes in contact with the pavement. It doesn't all "rise up out of the pipe very rapidly", - probably because it's condensing. If you've been on the streets in very cold weather you will see it and my guess is that it's more than you realized. Enough to cause problems over the course of a rush hour.
For ice to form you need water, - which has to come from someplace. Black ice as I'm describing it is typically found at intersections on dry, very cold days. Again, I'm not talking about bridges. Where does the water come from? Why intersections more than anyplace else? What do you suppose is happening to the water coming into contact with the pavement in the picture above?
Don't know about sidewalks but the fraction of my human breath that ends up inside my goggles can ice up them up on an extremely cold day, - and it's not Rime ice. It's smooth. Rime ice does form on my balaclava though.
Simply google "exhaust" and "black ice". You can decide if you find any of those resources credible.