I think people probably don't pay too much attention to car exhaust because it's largely invisible. But the sheer volume of it near a major road is obviously quite large. At least they are not putting lead in gas anymore, but perhaps there are other substances in car exhaust that are not so great for children's developing minds and bodies.
If the same quantity of traffic were to pass through smaller roads the quantity of emission would be higher. Cars are more efficient cruising at highway speeds than in stop and go traffic. Gridlock causes WAY more pollution than interstates. If a car gets 25 mpg on the highway and 17 mpg in stop and go traffic it will put out roughly 1.5 times the pollutants when NOT on the highway, over the same distance traveled.
Building a highway will not increase traffic. It will simply re-route the current traffic to a more efficient route.
I wonder if the ultra-low-sulfur diesel being mandated for trucks and autos in this country will reduce the soot content in the exhaust? I'm not sure the enviros are taking this possibility into account when planning to sue ICC construction, as they intend.