You can't just compute 60% multiplied by US population, 20% of Americans are children. You also probably only need contact tracing in certain high risk geograp

You can't just compute 60% multiplied by US population, 20% of Americans are children. 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.