I'll have to dig up the link, but there was a study done in Boston that concluded that the particulates in the air, primarily from motor vehicles, was causing IQ damage to children that was comparable to that caused by lead paint chips.

So, if we get folks out of their cars, we'll get a smarter society. The problem is, most of the people who seem to be able to understand the value of minimizing motor vehicle use are already smarter than average.