Concentrations for each block are calculated as the median of surrounding roads, typically located within 50-100 m from the block center point (SI Appendix, Figure ). The geographic assignment of on-road measurements to census blocks involved a two-step process. First, concentrations were calculated for road segments: the raw longitude and latitude values associated with each 1 Hz measurement were "snapped" to 30 m road segment points, as described by . To give equal weight to each pass of a road segment and minimize the influence of transient extreme events (e.g., vehicle plumes), measurements at each road segment were averaged per drive pass: all consecutive measurements made at a single road segment were summarized as a single mean value, following methods described in . Long-term concentrations at each road segment were calculated as the median of those drive pass means, with the median statistic chosen to further reduce the influence of outlier events. Second, census block concentrations were calculated as the median of concentrations at every adjacent or intersecting 30 m road segment, using 10 m buffer to capture road segments a small distance from the census block edge. Blocks vary in size and shape but typically cover an area of ~0.01 km 2 and are surrounded by roads, with a median perimeter of 447 m. Accordingly, census block estimates integrated measurements from 15-20 road segments (SI Appendix, Figure ). In some cases-near highways and strong point sources-pollution gradients may vary over finer spatial scales than those captured by census block spatial units (~110 m). However, the integration of multiple road segments provides an increase in total number of visits and total sampling time per spatial unit which reduces sampling error and measurement uncertainty . While on-road measurements are not a perfect approximation of concentrations throughout a census block, a previous comparison of on-and near-road measurements in West Oakland showed no evidence of bias in on-road concentrations due to increased proximity to on-road emissions . The general spatial representativeness of on-road measurements holds especially in low-traffic residential neighborhoods with mixed wind conditions .