The Manhattan intersection where a young cyclist was killed by a tractor-trailer is the most dangerous for bike riders in the Big Apple, according to a new report.

A total of 21 cyclists suffered injuries in crashes with cars at the corner of West 23rd Street and Sixth Avenue in Chelsea during a five-year period ending in 2018, according to the home-search Web site Localize.city.

Last month, bike messenger Robyn Hightman, 20, was riding north on the avenue when she was fatally struck by a turning big rig.

“Twenty-third street is wider than streets parallel to it, and it’s the main east-west route for delivery vehicles . . . this multiplies the risks,” said Localize.city urban planner Sam Sklar.

Sklar shared the data with The Post amid Mayor Bill de Blasio’s promised bike-safety initiative, which will include the redesign of 50 crossings with high bike injury and fatality numbers by the end of 2019.

De Blasio did not reveal which crossings would be targeted, but city data show that 60 percent of cyclist deaths since 2014 occurred at intersections.

City transportation officials put a protected bike lane on Sixth Avenue in 2016 — but it wasn’t enough to save Hightman.

“It appears the city is doing what it can, but we need to do a better job of training drivers, especially truck drivers, on how to look out for cyclists here and across the city,” Sklar said, adding that installing barriers at the southwest corner, where left-turning vehicles turning on to 23rd Street, would further boost cyclist safety.

Earlier this month, 28-year-old sculptor Devra Freelander was fatally struck by a cement truck in Williamsburg while on a bike.

“Brooklyn is where it’s actually dangerous. Drivers there never follow the speed limit, and neither do the bikers,” said a pal of Freedlander’s, Billy Maia, 30, a musician and part-time bike messenger.

“My friend, it’s so sad what happened to her, but she was running the red light. Accidents happen when you aren’t careful.”

The intersections of Jay and Tillary streets in downtown Brooklyn and Atlantic and Bedford avenues in Crown Heights were tied with the second-mont injuries with 20, according to the Localize study.

The former is a busy area for car and bike traffic, including those coming on and off the Manhattan and Brooklyn bridges.

“While some bike lanes are marked, such a major bike route need to be protected with better marked lanes,” said another Localize planner, Dan Levine.

Meanwhile, according to Sklar: “The bike lane on Bedford Avenue is not separated from traffic and is wedged between car-travel lanes. It’s unprotected on either side, and as riders cross the six-lane Atlantic Avenue, they’re expected to maneuver toward the curb. But often cars in a left-turn lane actually turn back into traffic, creating a risk for cyclists.”

City cyclists say many of the crossings on Localize.city’s list are in desperate need of changes.

“What the city needs is regulation and transparency, and some method of enforcement,” said 35-year-old Bronx resident Daniel Suh.

Fellow rider Ari Baum, 27, said the city is moving in the right direction but “it’s going to take some big changes to get things right.”

“Some very uncomfortable changes for car drivers,” Baum added, “but it’s happening.”

Additional reporting by Daniel Cassady