A moment last Monday, just after noon, in Manhattan. It's slightly overcast, not cold, a good day for walking. I'm in the 90s on Fifth heading south, enjoying the broad avenue, the trees, the wide cobblestone walkway that rings Central Park. Suddenly I realize: Something's odd here. Something's strange. It's quiet. I can hear each car go by. The traffic's not an indistinct roar. The sidewalks aren't full, as they normally are. It's like a holiday, but it's not, it's the middle of a business day in February. I thought back to two weeks before when a friend and I zoomed down Park Avenue at evening rush hour in what should...