Ashley is right. 'Cars' are countable, so it should be 'fewer cars', not 'less'. Colloquially, many people do use 'less' with countable nouns, but this is not grammatically correct. Also, you need a capital letter at the beginning of your sentence. Capitalisation is not an optional nicety - it is an necessary feature of the written language.

I would say air pollution will be less serious with fewer cars. The rule is that when the thing is a singular use less and when it is plural use fewer. You would say less water but fewer glasses of water You could say less tomato sauce but fewer tomatoes Reference:http://www.quickanddirtytips.com/education/grammar/less-versus-fewer

Yes, it's fine.