The Brooklyn-Queens Expressway was built in sections from 1944 to 1948 as part of the planner Robert Moses’ expansion of the city’s roadways, which helped create Interstate 278, the only interstate to pass through each of the city’s five boroughs. Under the original plan, a trench was to run through the middle of Brooklyn Heights, which would have required razing hundreds of historic homes and divided the neighborhood, said Peter Bray, executive director of the Brooklyn Heights Association, a nonprofit that rallied opposition to the plan.

As a compromise, the expressway was rerouted to the edge of Brooklyn Heights, though it still cut through other neighborhoods. It was also built into the side of a bluff, using an unusual triple cantilever bridge structure that allowed for a promenade to be added at the top. “We weren’t bluffing Robert Moses, but we had a real bluff,” Mr. Bray said.

While the promenade is prized by the neighborhood, Mr. Bray said the expressway is not. In some years, his association receives dozens of complaints about congestion, pollution, and noise and vibrations from cars hitting potholes and bumps. “I don’t think anyone loves the B.Q.E.,” he said. “We’re resigned to the fact it’s there.”

This year, the Transportation Department hired consultants to conduct the most extensive analysis of the highway since it was built, at a cost of $8.1 million. The highway has become increasingly congested, carrying an average of 45,300 vehicles a day more than it did in the 1970s, according to traffic records.

Inspectors drilled openings into the concrete walls of the triple cantilever bridge so that they could step inside and take a look. It was not a pretty picture. Though the highway was stable, steel rebars inside the concrete had corroded from road salt seeping into cracks. These cracks were getting wider from freezing and thawing, and moisture. Eventually the rebars will break apart, further weakening the concrete, and probably requiring emergency repairs, said Tanvi Pandya, a senior program manager for the department.