Add Falling Ice From Glass Towers to a New Yorker’s List of Worries

His car was totaled, but Mr. Nadler, fortunately, was not inside. “I would have been traumatized for life,” Mr. Nadler, 20, said on Friday. “It would have totally killed some people.”

An object that falls from a building, such as a piece of ice, does not have to drop from tremendous heights to accelerate to dangerous speeds. Frank Moscatelli, a clinical professor of physics at New York University, said that ice could reach its maximum speed, between 60 and 70 miles per hour, from the top of a 15-floor building.

It would travel the same speed at that height as it would if it fell from the tip of the city’s tallest building, 1 World Trade Center, whose spire climbs to 1,776 feet, Professor Moscatelli said. That’s because of terminal velocity, the maximum speed of a freely falling object.

But whether that ice bounces off a pedestrian below or crushes him depends on the size of the ice. “When the object hits you at, say, 64 m.p.h., it matters if it’s a penny or a brick,” Professor Moscatelli said.

On Feb. 4, 1994, Jill Gardenfeld, then a 24-year-old advertising analyst at Macy’s in Herald Square, was knocked unconscious by a 2-foot-wide ice slab. The jagged piece fell 21 stories, or roughly 300 feet, and struck her in the chest and shoulder, nearly killing her.

Ms. Gardenfeld survived, but spent two months in the hospital, underwent 14 operations and received 114 units of blood, her family told Newsday. She never regained the use of her left arm.

In 2014, frozen piles of ice and snow broke off 1 World Trade Center and plummeted more than 1,000 feet onto the busy walkways below. An entrance to the PATH train station from Vesey Street had to be closed on multiple days.