The first cousin to a bridge is an elevated road. For years, one of the most famous and heavily traveled was New York's West Side Highway, a Robert Moses creation that gave the automobile driver a view of the splendor and squalor of the Hudson River and the Jersey shore beyond.

But the steel under the West Side Highway went unpainted, and surface potholes went untended. The inevitable result came Dec. 10, 1973. A city highway department truck was rumbling along near Gansevoort Street when a large hole opened and the truck plunged to the ground.

No one was seriously injured, but that kind of incident attracts attention. It was not long before the West Side Highway, from 59th Street south along the Hudson River to the Battery at the tip of Manhattan, was closed. The rot in the steel and concrete was so bad that nothing was left to repair, and remaining sections of the West Side Highway have been removed.

Environmentalists and road builders are embroiled in the U.S. Court of Appeals here on whether to proceed with a grandiose plan to build a newer, better West Side Highway called Westway. The official federal cost estimate is about $2 billion which, for Westway's 4.2 miles, would make it $90,187 a foot, history's most expensive transportation project. The unofficial cost estimate is $4 billion.

Meanwhile, north of 59th Street, the continuation of the West Side Highway is called Henry Hudson Parkway and brings thousands of commuters into Manhattan from Westchester County. It, too, is falling down.

A state transportation department official led a tour recently and pointed to dangling pieces of steel, punctures in the concrete deck and one extraordinary, massive concrete and steel pier, cracked and incapable of continued support. Wooden blocks had been inserted to hold the deck. Repair plans are being drawn hastily.

Things are little better on the East River highway, the Franklin D. Roosevelt Drive, which also has many elevated sections. A section in lower Manhattan is being replaced. Uptown, the road is succumbing to constant battering from the East River not too far from Gracie Mansion, the mayor's residence.

But where to put the traffic while the road is replaced? On the graceful pedestrian walk within Gracie Mansion's boundaries? On the quiet residential streets of the elegant Upper East Side?

City and state have not solved that problem; reconstruction of the uptown section is to begin in 1984. It has been rescheduled, by necessity, from 1987. --Douglas B. Feaver