Exposure to traffic-related air pollutants, which has been associated with a range of adverse health effects, often is represented using indirect proxies or surrogate exposure measures, most commonly, the proximity to busy roads. This study examines the proximity of grade K-12 schools to high traffic roads in Wayne County, Michigan, an area including the industrialized city of Detroit as well as outlying urban and suburban communities. Unlike earlier studies, commercial and non-commercial traffic is distinguished, and effects of school type (public, charter, private), socio-economic variables, demographic factors, and mapping errors are evaluated. We find that total traffic flow, as measured by annual average daily traffic (AADT), does not reflect the substantial differences between trucking and commuting routes. Thus, AADT alone may inadequately capture traffic-related exposures, especially given the large differences between diesel and gasoline emissions. Based on close proximity (school-road distance < or = 150 m) to heavy traffic (AADT > or = 50,000), 4.9% of the 845 Wayne County schools are traffic exposed at school. In the urban core area, 7.2% of schools and 7.6% of students are traffic exposed at school. A larger proportion of grade 7-12 students in public schools are exposed than K-6 students. Considering truck emissions, 2.8% of the schools are within 150 m of roads with 5000 or more trucks per day. In Wayne County, students attending schools near high traffic roads are more likely to be Black or Hispanic, to be enrolled in a meal program, and to reside in a poor area. Many of these results are driven by the large minority population in the densely populated core area of Detroit. The findings show that a large fraction of children have high exposures to traffic-related pollutants, especially in Detroit, and the need for exposure measures that account for both the composition and volume of traffic.