Masks may be able to help a little, but odds are good that if you're in a bus with an infected person coughing or sneezing, you'll be infected with or without a (regular surgical-style) mask. Those masks are almost entirely designed to keep stuff in, not to keep them out of your nose/mouth/eyes.
The slightly reduced incidence of infection from direct contact would not at all affect the greatly increased incidence from much more frequent direct contact. Which is why no country is relying on it.
“Keeping stuff in” is exactly the point. Covid-19 causes mild symptoms for most, so many people walk around infecting others without even realising it. Where there is widespread use of masks, transmission is greatly slowed/reduced.
The point of wearing the mask is not only to protect yourself, but even more so to protect others in case you are infected.
I feel the crux of Japanese resilience (vis a vis Western) has been their tendency to consider and act in the group's interest, rather than in own interest.
However, it's far from over and time will tell. Yesterday I saw many people doing hanami, cautiously.
It seems like that would cost less, and might also be more effective than shutting down half the economy?