Exhaust from cars add only little to air quality. Trucking is more important (I've read somewhere a freeway with 10,000 trucks emit the same as a freeway with 100,000 cars). In Europe, road traffic is generally responsible for about 10% of the emissions, although it varies by type (for example, NOx emissions from vehicles is worse than PM10 or CO2). I guess it would be somewhat higher in the US because there is somewhat more traffic and a larger share of the vehicle fleet has a relatively low gas mileage.
I conduct air quality surveys for work, using computer models. Generally, the background concentrations is around 80 - 90% of the PM10 (particle matter) concentrations, and the rest is added by a freeway (I'm talking about major freeways, say 100,000 vehicles per day).
Air quality is predominantly influenced and fluctuated by weather patterns and geography, this is why Mexico City and Los Angeles have worse air quality than cities in flatter terrain with a sea breeze that is not trapped by mountains. Although it has to be mentioned air quality in greater Los Angeles improved massively since the 70's, while the number of cars exploded in the same time.
In overall emissions, road traffic doesn't have that much influence, although there is some impact on local air quality, for example within 600 feet off a major freeway, especially when there is a bottleneck. Queuing traffic emits 3 to 6 times more than free-flow traffic, most notably trucks. That is why a road widening is often positive for local air quality, even if there is more traffic than in the old situation.