A study of outbreaks (where three or more people were infected in one go) in China showed that ~80% were home outbreaks (HT: Marginal Revolution). Transport was second, involved in 34% of the outbreaks (the same outbreak could be in multiple categories).
The same study showed that all outbreaks involving three or more cases were all from indoor transmission, which may be an indication that getting outside might be good.
This might also support the theory that stay-at-home orders caused the initial peaks of cases many places experience.
This theory doesn’t say that stay-at-home orders cause more cases. Rather, it just condenses a week or two worth of new infections into a 1-3 day window after the order goes into effect as those with the virus become more likely to spread it to their families.
This essentially creates a “rush hour” of cases 2-3 weeks later, that was mistook for an exponential growth of the virus.
Consider how viruses typically move through your household. It’s rare for everyone to get it at once. Part of that is driven by incubation period and previous immunity. But, some of that may be other factors like how often you cross paths and for how long.
In a normal day of work, school, soccer practice, meetings, travel, TV and homework, you might have an hour of face time with your family, which might lower the chances of spreading something you are infected with to them.
But, when everyone stayed at home at the same time, you are with them longer. And, if you have the virus, the chance of you spreading it to your family in the days after the order goes up considerably.
If true, you might expect to see less severe initial peaks in areas that did not panic and lockdown at some point. But, those areas are becoming more scarce.