It is a fascinating study of walkability and physical activity. It is interesting and expected that the association between walkability and physical activity changes among SES. I think that it is expected because people with high SES are more likely to live in better neighborhoods, with better walkability score, but also people with high SES are more likely to have a car, and therefore they don't walk or use public transportation to go to work/school. I am wondering if the authors did an analysis identifying the type of physical activity: maybe high SES have a higher prevalence of recreative physical activity (i.e. go to the gym or practice some sport), while low SES have a higher prevalence of physical activity due to active transportation.