Could many small cars emit less NOx and other true pollutants than public transport vehicles?
Thanks to catalytic converters almost all vehicles put out only a tiny fraction of what past vehicles did. This includes diesels, heavy trucks etc. So it could be possible one bus may put out less emissions than 20 cars. But mainly that one bus burns much less fuel than those 20 cars combined. Lastly a number of public buses run on natural gas which is a lot better than gasoline or diesel in terms of emissions.
