See how the Census Bureau calculated the risk here.
Several thoughts on the map:
1. There are some patterns. Many counties in the Northeast, Midwest, and Mountain West having lower levels of risk (outside of urban counties). In contrast, the Sun Belt and the West Coast are at higher risk.
2. Doing this at the county level makes some sense: people might travel within a county for work, errands, and other activity. At the same time, there could be significant disparities across communities within counties that a map like this covers up. For example, it looks like DuPage County is somewhere in the middle and some suburbs there have higher rates of cases than others.
3. The differences across geographies also speak to the difficulties of enacting policies across different areas. On the risk map, some states have more consistent patterns while others have a mix (ranging from more equal numbers of counties at different levels or a few heavily populated counties versus everyone else). National policies have to address places with different experiences and different futures.
4. This data cannot tell us how many people cross county lines within a region or state. There are clusters of counties with similar risk levels but this may not speak much to travel as to similar populations.