I visited both of these locations in recent weeks and was intrigued to see the mix of people at each. I’ll make a quick case for why these locations could provide as good cross-section of America as any other location:

1. Limited options. For the emergency room on a Saturday night, there are few other medical options available at that time. If anyone has a medical issue, they will end up here. As for Walmart on Sunday evening, there are limited brick and mortar shopping options and the work week is about to start.
2. People need medical care and grocery/home items. Both locations have people trying to meet basic human needs. Even as online shopping may allow people to avoid other shoppers and online medical consultations are now available, there are inevitably moments where running out to a store or medical professional is necessary. It is hard to imagine either of these facilities disappearing completely (even if the number of retailers is severely reduced).
3. Connected to #1 and #2 above, people of differences races, ethnicities, and social classes are at both locations. In many other locations, whether due to residential location, the location of jobs, ill will toward others, or access to resources, not all groups are represented. Sociologist Elijah Anderson wrote a book about such rare urban locations.

While these may not be the best locations in which to conduct research, they could offer insights into typical American life.