I believe a gallon of gasoline, when burned and combined with the oxygen in the air, produces about 20 pounds of carbon dioxide. My commute, if I drove, would require half a gallon of gasoline based on the distance and mileage of the car. Therefore, each day I commute I refrain from producing ten pounds of carbon dioxide, plus the other by-products that I know nothing about.
I saw an article on the internet some years ago that tried to prove, through calculation, that one biker would cause more pollution with his bike than he would with his car. The reasoning was that on an average trip a certain number of automobiles would have to pass him, and to do so they would have to slow down, wait for the opportunity, move to the left a bit, pass, and then get back up to speed. The getting back to speed would burn some extra gasoline, and the extra gasoline burned by all those people passing the bike would make up for the gasoline not burned by the bike.
I think you can prove just about anything with enough effort.