From experience cycling in london cars don't belch as much nasty crap into the atmosphere and its the big vehicles buses and lorries lorries turning into you that are the real killers.
Busses here have been natural gas for a while.[1] We don't have many lorries here, mostly very large 18 wheelers and local deliver trucks...I count the delivery trucks amongst the regular car drivers. I wouldn't say they're more dangerous, though not less.
