Nivardo Lopez, 36, the transportation department’s Bronx commissioner, said that when he was growing up in that borough, he used to go to Manhattan every week to visit relatives and friends, and to shop and eat along Dyckman Street. Though he lived near the University Heights and Broadway bridges, back then it never even occurred to him to walk. “Those bridges are what connect these communities, and for years, cars were the only safe way to cross,” he said.

Gale Brewer, the Manhattan borough president, said that the crossings not only provided a free alternative to subways and buses for those on a tight budget, but were also better for the environment and gave people another chance to squeeze in some exercise. “We should be walking, we should be bicycling, as opposed to getting in a car or subway,” she said. “It’ll be healthier. We’ll live longer.”

The Madison Avenue Bridge, which was built in 1910, takes its name from a Manhattan street synonymous with the city’s advertising industry and tony designer boutiques. The bridge links East Harlem with Mott Haven, in the South Bronx, both communities whose residents have long struggled with poverty, asthma and obesity-related diseases such as diabetes.

Though it will still be years before bike lanes are installed on the bridge, new protected bike lanes have recently been added on the Bronx side on East 138th Street for several blocks. Mr. Torres, the East Harlem teenager, said he cannot wait for the bike lanes to run all the way across. “I’d be able to go more often and get there safe,” said Mr. Torres, whose favorite deli is in the Bronx. “A bike lane would be really convenient.”

City transportation officials have also made other improvements to that stretch of East 138th Street in the Bronx, where there have been a spate of vehicle crashes involving pedestrians and cyclists in recent years. A few blocks down from the bridge, a new intersection with traffic signals and crosswalks has been created across from a busy subway station for the No. 4 and 5 lines. Giant planters — and soon tables and chairs — sit in a crescent-shaped curve on the street where livery cars once parked illegally.