This is a problem we should have addressed some time ago because it is a problem in relation to many more things than COVID-19. It is understandable that people may enjoy the feeling of being in a crowd, and being a part of the gigantic workforce that keeps the economy moving forward, but you'd probably get the same feeling in New York City if the crowd was a quarter of the size that it gets during rush hour. And, if rush hour traffic was quartered, there'd be a smoother communte, a lot less gas wasted by idling cars, a corresponding drop in air pollution, a lot less stress, and so forth. So why don't we do it? The only thing keeping rush hour alive is our silly adherence to the 9 to 5 workday--in a global economy, that makes no sense at all. So let's stagger start times (and lunchtimes and quitting time will stagger as well) so that ¼ of the workforce starts at 8, another ¼ starts at nine, another at 10, and a fourth at 11. At noon, a round of lunches begins, at four, a round of departures. Everybody gets a full day in, but we have ¼ of the workforce on the street each hour: a more manageable crowd.
Who arranges it? Well, as a start, businesses can voluntarily stagger their start times, or divide up the arrival times of their employees so that ¼ arrive at the designated times, but the local government will probably have to put the finishing touches on things to even things out.
So that much just addresses the madness of rush hour. Next, to deal with the COVID crisis, we have to develop a convention for walking on sidewalks, that is not much different than what we do with cars: put the cars going in opposite directions on opposite sides of the street. Before we get into that, however, let's imagine a crowded city sidewalk, jam packed with people. Now, remove 7/8 of them. That crowd thinned out pretty quick, didn't it? That's what we achieve when we divide the workforce into fourths, put ¼ on the street, and then put half of that group, (1/8), on one side of the street, and the other half on the other side of the street. But, let us not forget the cross streets, which also have two sides; split the ¼ on the street by the four options that a grid allows, and the crowd on our sidewalk initially is whittled down to 1/16 its original size—15 out of every 16 people on that sidewalk are no longer there. Starts to look like Social Distancing, doesn't it?
Now, let's take the next step, and ask people in the Red 1, Red 2,Green 1, and Green 3 groups to stay on one side of the street (keeping Social Distance from Red 2 people), and ask those in the Red 3 and Yellow 2 groups to stay on the other side (keeping Social Distance from each other).
So that's a good start, but intersections will pose a problem, and things will get difficult if a Yellow 2 guy wants to go to a shop on the Red 2 side of the street. How can we work this out?
Again, we have to develop some new conventions because it's no longer reasonable for people to crowd together at corners the way they used to. So, again, we have to learn a lesson from the cars. They line up in more or less good order, keeping a safe distance from one another (not jostling each other, or shoving their way to the front), and wait for the light to change. People will have to do the same thing, keeping a good 6 feet apart, and moving forward as the traffic light allows.
In order to keep the groups apart, how we cross the street will have to change as well. The way we currently do it, two crowds head straight at each other, and everyone dodges and weaves as best they can. The pickpockets love it. The way to avoid this is to cross as if the intersection is a roundabout (a.k.a., traffic circle or rotary). This gets everyone moving in the same clockwise or counter-clockwise rotation, and nobody comes face-to-face with somebody else. The length of time the 'walk' signal is on will have to be lengthened, and people who are crossing the street will have to walk further about ⅓ of the time, but trading a bit of exercise for health, order, and fewer picked pockets is a good deal.
To deal with the problem of someone needing to cross to the 'wrong' side of the street, we can, first, practice enhanced distancing any time we encounter someone in such a situation. Secondly, however, we can minimize the occurrence of such events by switching which side of the street is the Green 3 side every day. Since there are 7 days in a week, any particular day (say Wednesday) will be available to a given group (say Red 2) once every two weeks, while any particular side of the street should be available to them every other day. Most people should be able to schedule things accordingly.