Taxis and buses (diesel) produce the most particles.

http://www.businessweek.com/articles/201...uel-blamed

IMO, it's also helpful to try and keep traffic flowing in a very busy city, rather than frustrating it with poorly timed traffic lights etc... as this reduces the amount of time engines are on overall and reduces the time cars are sat in one place burning fuel.

That said, as a pedestrian, I'd rather cross the road, than cross halfway across the road and wait again.