Air pollution, of which transportation is a huge contributor, kills tens of thousands per years in the US. There are many other effects like asthma which afflict millions.
Transportation is certainly a lot cleaner than it was but there's still a lot of improvement that can be done. Below is a link to a basic overview, it can give you hints on where to dig if you are interested in specific studies.
I just looked up the brake pad emissions. Pretty scary stuff but it seems like it would be trivially solved by some form of containment around the brake and the disc that captures all of the dust. Probably needs laws to enforce it before car makers would add such a thing.
