Privacy preserving contact tracing: the comic
ncase.me
ncase.me
[1] https://science.sciencemag.org/content/early/2020/04/09/scie...
So... 200 million americans?
You also probably only need contact tracing in certain high risk geographies, urban centers, and a few suburban centers.
60% of the remaining can definitely fall under the "knowledgeable, willing, personally able (authorized to install app), technologically able (app works with their devices), and vigilant". Even if you don't hit 60%, you're still reducing R0 by enough that you can carefully start to re-open certain parts of the economy.
Only concerned because of (not personal) past experiences.