SAN FRANCISCO — We’re in week four of sheltering in place here (it feels like week 40). It’s a completely unfamiliar situation in so many ways. As someone who has lived in cities her whole adult life, for me it’s especially strange to experience a time when all the things I love are no longer available. Nearly everything is closed — restaurants and shops, libraries and museums, and of course all schools. All nonessential workers are under a mandatory work-from-home order.

But these efforts to stem the spread of the coronavirus have also offered us a rare experiment: We can see our cities for the first time without the choking traffic, dirty air and honking horns that have so often made them intolerable.

Throughout the world, the coronavirus has forced extreme changes in our behavior in just days. And we’re already seeing the impact of those changes: On Monday, for example, Los Angeles had the cleanest air of any major city in the world.