During the last several decades, industrialization and urbanization have resulted in dramatic increases in vehicle-associated emissions. More than 50% of the total particulate matter (PM) emissions in urban areas are related to road traffic (Briggs et al. 1997). A number of studies have reported significant associations between traffic density or proximity to roads and various respiratory symptoms in children (Brunekreef et al. 1997; Edwards et al. 1994; Venn et al. 2001).