Thanks Tyler,
I've read of studies that determined that living near major roads has a negative effect on measures of intelligence in children. I go the impression that they were talking about the steady noise factor. But I guess it could be argued that this might be that the parents could not afford to live in nicer neighbourhoods and so were possibly not aware of the importance of education or proper nourishment etc.
Another big factor is the carbon monoxide factor. Once it is breathed in it takes a long time for the body to get rid of it as it binds so well with the blood.