It's cars, yes, and it's also other forms of transportation including railroads and airplanes, it's street cleaning vehicles, leaf blowers, lawn work, garbage c

It's cars, yes, and it's also other forms of transportation including railroads and airplanes, it's street cleaning vehicles, leaf blowers, lawn work, garbage collection, inconsiderate people, construction work, etc etc. Not all of these are going to avoidable, but at the moment it's a free externality, there is essentially zero incentive to cut it down and it shows.