Study areas were repeatedly visited on a rotating schedule designed to assess long-term average concentrations indicative of typical weekday, daytime conditions. During a visit, the driver would follow a Google Street View-based driving protocol to visit every road segment within the neighborhood at least once, driving with the normal flow of traffic. For large study areas, a sub-unit would be assigned for full coverage in a single day's driving, with full sampling occurring over multiple days. Visits to each area were distributed over different times of day and different seasons Incidental temporal sampling bias was assessed using fixed-site regulatory monitors located in five of the sampling areas (SI Appendix) Study areas and demographic data Thirteen urban districts, cities, and towns ranging in size from 2.4 -15.7 km 2 (total: 93 km 2 ) were selected as study areas to provide a range of land uses (e.g., industrial, commercial, dense residential, and light residential), share of open or green space, traffic density, demographic composition, and historical housing policy (SI Appendix, Figures , Figures S12-13 and Tables ). Study areas were distributed within the counties of San Francisco, Alameda, San Mateo, and Santa Clara in the San Francisco Bay Area, with one background location in Sonoma County (SI Appendix, Figure and Table ). Sampling areas include between 95 and 930 census blocks (median: 447), depending on population density and spatial extent (SI Appendix, Table ).