What is "known" has been evolving.
Best information I've seen lately is that surface transmission is much less of a risk than was previously thought and that close personal contact, particularly in closed spaces, is the most dangerous. (Hence rapid spread in places like nursing homes.) The most credible thing I've seen on masks is that masks have a minimal (but not zero) benefit for the wearer, but help reduce transmission from the wearer. In other words, we wear masks mostly to protect other people, as we can never know with certainty if we have been exposed and can expose others. The broad wearing of masks will result in reduced risk, which lowers the R factor of the spread. I suspect that masks in a car would be moderately beneficial, but the longer the ride, the more gets through the mask of the infected person--and a car is about as confined of a space as you can get. (Outside, what's in the air can disperse. And exposure isn't all or nothing--how much of the virus gets into your body is one of the critical factors as to whether you actually get sick or not.)
For me, based on what I've seen so far, I'm comfortable with a quick trip into an uncrowded public restroom--so long as I can wash with soap or hand sanitizer immediately afterwords and don't touch my face from the time I enter until I leave.