WASHINGTON -- The police surge in New York is intended to be a deterrent for a city that saw a car plow into a crowd in Times Square in May. The driver, believed to be under the influence of drugs or alcohol, killed one and injured 22.

Ron Hosko is a former assistant director of the FBI who has worked on securing major events, including the Olympics and political conventions. He said it's "almost impossible in an urban setting" like Washington D.C. to protect sidewalks from cars.

"I think there is a way with those high-concentration areas, and we see it outside of sports stadiums, we see it in other places, we see it right behind us, where it's a safer pedestrian area and tourist area and citizen area behind us than it is on the street," Hosko said.

The NYPD guard a Times Square sidewalk. CBS News

Terrorists have used the tactic successfully -- Thursday in Barcelona, last summer in Nice.

Those attacks prompted an increased security across the U.S. for major events like the Macy's Thanksgiving Day parade in New York and the opening of the National Museum of African American History and Culture in Washington. Some office buildings in D.C. have installed permanent pylons aimed at stopping a vehicle.

The Department of Homeland Security issued a warning to commercial trucking companies in May that terrorists may look to use, rent or steal their vehicles for attacks targeting pedestrians.

But it doesn't have to be a truck. On Saturday in Charlottesville, police say James Alex Fields Jr. drove his car into a crowd, killing 32-year-old Heather Heyer and injuring at least 19 others in a matter of seconds.

"There is no cost-effective way to secure every sidewalk in a major city -- there is none," Hosko said. "We can harden targets that are high concentration areas, but we're always going to be vulnerable."

Barricades could stop a car, but essentially wall off the sidewalk. It's open question if people would put up with the inconvenience.