Distances within 300-500 m of roadways affect human health. In large North American cities, 30-45% of people live near a major roadway. Patients with asthma should ideally live at least 300 m from major roadways, especially those with heavy truck traffic. In-vehicle exposure during commuting with open windows can also be very high. Air pollutants cause oxidative injury to airways, leading to inflammation, remodeling, and increased sensitization.
14% of incident asthma and 15% of all exacerbations of childhood asthma are attributed road traffic pollutants. Air pollution can cause exacerbations of pre-existing asthma but it also might cause new-onset asthma. Cities with rapid economic and population growth (China and India) have some of the worst air quality in the world. Short-term exposures to ozone, nitrogen dioxide, sulphur dioxide, PM2·5, and TRAP is thought to increase the risk of exacerbations of asthma symptoms.
This comprehensive review from The Lancet presents a snapshot of our current understanding of the relationship between outdoor air pollution and asthma (free full text after registration).
Outdoor air pollution and asthma. Michael Guarnieri et al. The Lancet, Volume 383, Issue 9928, Pages 1581 - 1592, 3 May 2014, doi:10.1016/S0140-6736(14)60617-6
Image source: Interstate 80, seen here in Berkeley, California, is a freeway with many lanes and heavy traffic, Wikipedia, GNU Free Documentation License.