Planners describe it as a turning point in the redesign of Times Square: a meeting around the end of 2012 when David C. Kelly, then the Police Department’s assistant commissioner for counterterrorism, showed up with a copy of Al Qaeda’s English-language magazine, which called on attackers to "mow down” pedestrians with pickup trucks.

Soon, at Mr. Kelly’s recommendation, pedestrian plazas that had replaced roadway along a five-block stretch of Broadway were guarded by three-foot-tall, stainless-steel cylindrical bollards.

The thick posts stopped a sedan last week from plowing into dozens more pedestrians after a man, high on PCP, drove unimpeded through three blocks of city sidewalk, killing an 18-year-old woman and injuring 20 other people, the police said. The rampage brought to mind the terrorist attack last year in Nice, France, in which a cargo truck killed scores of people celebrating Bastille Day, but the authorities in New York said that while the driver, Richard Rojas, was trying to kill people, he had a history of mental illness and that they did not consider it a terrorist attack.

The bombing on Monday in Manchester, England, was another reminder of ever-evolving terrorist tactics, and it prompted counterterrorism officials in New York and in other large United States cities to look into additional precautions.