The objective of this study is to quantify, using virtual audits in Madrid and Philadelphia, cross-city differences in the walking environment and to test whether differences vary by sampling method. We used two sampling methods; first, a contiguous area combining census units (~15.000 population area for each setting) was selected using the Median Neighborhood Index (MNI). MNI is a summary index that averages Euclidean distances of sociodemographic and urban form features, used to select the median neighborhood for a given city. Second, we selected a population-density stratified sampling of the same number of census units as above. M-SPACES audit tool was deployed, using street virtual audits to measure function, safety, aesthetics, and destinations along each street segment. Madrid streets had lower scores for function (b=-0.29 CI95% -0.55;-0.31) and safety (b=-0.38 CI95% -0.61;-0.14). Madrid had a greater proportion of streets having at least one walking destination in the street segment (PR=1.92 95% CI 1.55; 2.39). We did not find a significant difference between Madrid and Philadelphia in aesthetics. We found an interaction between safety and sampling methods. This approach can reveal which elements of the built environment account for between-city differences, key to mass influences that operate at the city level.