Bolted into the side of a Brooklyn Heights cliff, the triple-cantilevered Brooklyn-Queens Expressway today is about 10 years past its expected lifespan and starting to crumble.

That has experts like former city traffic commissioner Sam Schwartz worried. Crumbling can expose the highway's steel-mesh underbelly to corrosion, Schwartz recently told The Brooklyn Paper. And if that happens, lanes must be closed and repair work accelerated. For an artery that on a good day moves 170,000 vehicles through Brooklyn and Queens, this is a nightmare scenario.

Crumbling isn't the expressway's only problem. Meant to alleviate traffic jams on surface streets in Brooklyn and Queens, the BQE itself, from Red Hook to the Triborough Bridge, has become one long, sluggish mass of crawling motor vehicles during all but the wee hours of the early morning.

Built before the Interstate Highway System set national standards, the BQE's lanes are too narrow, and its entrance and exit ramps are notoriously short and dangerous. Meanwhile, as traffic edges around Brooklyn Heights, incessant vibration from large trucks causes structural problems in nearby homes.

Who knew? When it opened in 1954, the Brooklyn Heights segment was regarded as an aesthetic triumph and engineering marvel. The structure's top level became the celebrated Brooklyn Heights Promenade. It hid the expressway below and offered gorgeous views of the East River, Brooklyn Bridge and Lower Manhattan. The design allowed NYC master planner Robert Moses to route the BQE around Brooklyn Heights and not straight through it as originally planned. Red Hook and other neighborhoods weren't so lucky.

Today the Promenade remains one of the city's premier public spaces. But for now, no one plans to replace the slowly disintegrating highway beneath it. Not good. Government needs to act before salt air, corrosion, cross-country trucks and gravity take their toll.