The only problem is that it requires everyone to actively participate, which means every participant must be all of: knowledgeable, willing, personally able (authorized to install app), technologically able (app works with their devices), and vigilant. This is currently logistically impractical.
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.
This is great! Slowing the spread of the virus shouldn't have to mean sacrificing our privacy. And if it becomes widespread enough, we may be able to go in public a bit more, knowing that we'll be alerted if we've come in contact with a sick person.
