It's easily accessible on real estate websites (like Redfin, https://www.redfin.com/how-walk-score-works), and now I see there's a publicly available dataset as well: https://catalog.data.gov/dataset/walkability-index.
Walkability that is commonplace in many towns in Europe, roughly match up with very high 90's (98+) walkability scores in the US.
IMHO, anything below mid-90's is somewhere I would never live. The score is necessarily skewed towards US style living where walkability is relatively rare. So the difference between, say 70 and 40 is almost meaningless from the point of view of a walker.
As far as street design and urban planning goes, the key word is "complete streets" which is an umbrella term that covers rationale, design principles and practical guidance for how to implement a street layout (https://www.transportation.gov/mission/health/complete-stree...). This work is based on studies and data, but I've not looked into it.