Before dawn on Wednesday, Brian Levy, a union electrician, drove into work in Manhattan’s Financial District from his home on Long Island. The commute, normally clogged by traffic, took half as long—the roads were nearly empty.

From his perch working on the upper floors of 222 Broadway, Mr. Levy looks out on a silent city: darkened windows, deserted streets, shuttered businesses. Blocks south, as markets opened on Wednesday morning, an empty cobblestone plaza gave the New York Stock Exchange the air of a mausoleum. There...