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.

https://www.ucsusa.org/clean-vehicles/vehicles-air-pollution...