Recent studies suggest that spending a significant amount of time in proximity to vehicular traffic has adverse effects on human health, especially to vulnerable populations such as the elderly and children. Concentrations of many traffic-related pollutants decrease rapidly with increasing distance from the road, underscoring the fact that fixed-site monitors for air pollutants in communities may not capture traffic-related exposures important to human health. Few traffic-related pollutant measurement studies have been conducted in developing nations where traffic-related pollution may be higher due to older vehicular fleets and other conditions. In the present study, roadside pollutants were measured at 55 sites in the city of Tijuana, Baja California, Mexico, including 18 in proximity to elementary schools. The relationship between traffic counts and air pollutants was analyzed by performing 20 min simultaneous roadside measurements of black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (PAHs), fine particulate matter (PM2.5), ultrafine particle number (UFP), and carbon monoxide (CO). In addition, some 24 hr passive measurements of nitrogen oxides (NO_) were conducted. Traffic volume and flow was assessed by video recording the measurement session at each site and performing subsequent traffic counts. Most pollutant concentrations were highly variable across different areas of Tijuana. The median BC concentration was 4437 ng/m_ with the highest levels measured in the SE area of the city. The median concentrations for UFP, CO and NO_ were 30,265 particles/cc, 1.6 ppm, and 49 ppb, respectively, with the highest levels measured at the "5 y 10" intersection. The median counts for autos and trucks were 1758 ct/hr and 180 ct/hr. Ultrafine particles number concentrations were most highly correlated with traffic counts, as was BC to a lower extent. PM2.5 was least correlated with traffic counts. Carbon monoxide concentrations were highly correlated with both ultrafine particle number and BC concentration making CO a possible simple surrogate for near-traffic exposure measurements. At school-related sites, the median UFP number and BC concentrations were 3673 ng/m_ and 28,566 particles/cc. In addition, UFP number concentrations were significantly higher under moderate or intermittent traffic flow conditions. Finally, there was some evidence that low socioeconomic status (SES) areas had higher CO levels. However all other pollutants were not significantly different by SES status indicating that traffic-related pollution represents a hazard to many city residents. These findings suggest that traffic related-air pollution may affect public health in Tijuana, and that air quality interventions to reduce traffic-related emissions and policy measures to route traffic away from schools are warranted.