.I have this idea that as long as there are cars around your veggies get coated in the soot.

Depends. There's no way I'd have eaten anything that grew in the basement garden: it was in a nightclub area and people used to provide, um, liquid feed on occasion, though less so after we planted it up - I guess they realised it was a residence and pissed on the office next door instead. On top of that there was a lot of traffic on the road outside, which was  a major artery into the city centre: trucks, buses, cars all  stuck in traffic jams ten hours a day.

The air around our place is ok enough for lots of lichen growth, and given that bought in food passes through trucks, loading docks and so on, not to mention whatever is sprayed on it, I figure it's not going to make much difference.