JERSEY CITY — Class had just let out on a recent Monday evening at JC Taekwondo and Kickboxing Academy on Newark Avenue. Children wearing bright white keikogi practiced their moves on the sidewalk and chased each other through the street. They darted around families pushing strollers, diners sitting at painted picnic tables and weary commuters trudging home from the Grove Street PATH station a block away.

The would-be ninjas’ training helped them weave and bob through the crowd, but there was one danger they did not have to dodge: traffic.

Flying in the face of a century of development — to say nothing of New Jersey’s identity — the city closed Newark Avenue between Grove and Erie Streets to cars and trucks as of Aug. 19. The closing, which is in effect from 3 p.m. until midnight on weekdays and throughout the weekend, is a trial effort set to run through the end of October.

The move to shut down a blocklong stretch of downtown Jersey City’s main thoroughfare is just one of many steps that Mayor Steven Fulop’s administration has taken in an attempt to help tame the streets of the state’s second-largest city.