By PETER C. BELLER
ROSLYN

FOR 56 years, it has been a by­-pass. For the next 30 months, it will be a bottleneck.

The Roslyn Viaduct, the nondescript and little-loved half mile of rusting steel and worn concrete that carries Route 25A over and past the village of Roslyn at the head of Hempstead Harbor, is an aging relic of Long Island's postwar highway­ building boom.

Route 25A, known here as North­ ern Boulevard, is the only east-west through road on Nassau's hilly, bay­ indented North Shore, and the via­ duct is a key section, traversed at highway speeds by 38,000 vehicles a day.

But it has no shoulders, its sight­ lines are poor, heavy trucks are banned, and turning on or off the road near the bridge often requires stepping down hard on one pedal or another.

And the viaduct is certainly no monument to structural engineer­ ing: its obsolete pin-hanger design, with concrete piers propping up steel sections linked together with metal pins, is similar to that of the Mianus River Bridge on Interstate 95 in Con­ necticut, which collapsed in 1983, state and local officials said. New York State stopped building pin­ hanger bridges in 1968.

So it is without nostalgia, or some­ times even much awareness, that Roslyn residents are greeting the re­building of the hulking structure that dominates much of the landscape in their historic village.

Last month, after years of plan­ning, the state began work to demolish and replace the viaduct, a $127 million undertaking that is scheduled to last into the spring of 2008, according to Eileen Peters, a spokes-woman for the State Transportation Department's Long Island regional office.

"It's basically outlived its useful life," Ms. Peters said of the viaduct, which opened in 1949. "It wasn't built for today's volumes of traffic."

After the Mianus collapse, which killed three people, the Roslyn Via­duct was reinforced with metal slings meant to prevent a similar dis­aster, said Mark Bocamazo, the re­gional design engineer for the department. But there comes a point, he said, "where you either have to do?

Vie DeLucia for The New York Times

The bridge carrying Route 25A over the Roslyn village waterfront will be replaced in stages, with three narrow lanes kept open to traffic.

significant improvements to bring it up to current structural standards, or you make the judgment it's more cost effective to replace the bridge entirely."

The new viaduct - actually a side-by-side pair of viaducts - will be of  modern steel-reinforced concrete box-section construction, with a roadbed 13 feet wider than the old viaduct, to accommodate shoulders and sidewalks.

Entrance and exit ramps, drain­ age and lighting will be improved to meet current standards. And there will be fewer, more widely spaced piers, opening up more land beneath the viaduct for other uses.

A vital road link, but an obsolete eyesore.

Preparatory work will occupy construction crews for several months, as they clear ground and excavate for new foundations. Work on the roadway itself - with the resulting traffic disruptions - is due to start in February.

With no practical detour available, state engineers have planned the project to keep one side of the ex­ isting bridge open while the other half is demolished and replaced.  Three narrow lanes will be squeezed into the space of two; during the morning rush, two lanes will be open westbound and one east, with the pat­tern reversed in the evening.

Under those conditions, an acci­dent or breakdown on the viaduct would tie up traffic for miles, so vid­eo cameras will be set up to enable transportation department workers to monitor the viaduct when crews are not working, and a tow truck will be on call 24 hours a day to remove disabled vehicles quickly, Ms. Peters said.

To many, the presence of a huge el­evated highway fencing off the heart of Roslyn from Hempstead Harbor is a Robert Moses-style trade-off typi­cal of Long Island's automobile-dom­inated landscape. Before the viaduct was built, through traffic had to squeeze through Roslyn's congested main business street, now known as Old Northern Boulevard, and negotiate several turns at each end of town.

The viaduct was the price the vil­lage paid for relief from the through traffic, and the inconveniences of the reconstruction are seen in the village as equally unavoidable.

"Probably, most people consider it an eyesore," said Cecil Pinder, 78, the president of the Roslyn Land­ marks Society. "But practically, it was inevitable, if you were going to use 25A/Northern Boulevard as a major artery.

"They are somewhat resigned to the fact that it is a necessity, apparently," Mr. Pinder said of his fellow villagers. "It's not a topic of general conversation." ,

Complaints about the work have been few so far. Some residents have raised concerns that trees cut down by work crews during the project will not be replaced. And the Trinity Episcopal Church objected to giving up a portion of its property at the east end of the viaduct for the project;  the state used its eminent domain power to acquire the land.

Donald Kavanagh, a member ( Trinity's vestry, said that the church grounds had been whittled away by the state before, to widen Norther Boulevard, and that the rumble ( passing traffic had become a problem for the brick church building, landmark designed by Stanford White with Tiffany stained-glass windows.

"We've got an important SSE and we just don't feel the state has been very sensitive," he said.

Nearby businesses are also annoyed at side effects of the project saying it has aggravated traffic ar parking problems in the village.

A 40-space parking lot under the viaduct's eastern end, leased from the state by Diane's Bakery and formerly used by customers of several nearby businesses on Bryant Avenue, is now filled with constructic equipment. Other parking spaces across the street have been blocked off so that heavy machinery can maneuver in and out of the lot.

And to top it off, parking limits: meters that were recently installed along Bryant Avenue are being zealously enforced, with $30 tickets ( violators, according to Jerry Bali owner of the Dresser, a men's clothing store a few doors from Diane's.

"For the first time in 19 years, IT customers are complaining to me Mr. Balin said. "Business is challenging to begin with, and this has just made it a little bit tougher."

The mayor of Roslyn, John Durkin is one of the owners of Diane's. While the lot under the viaduct is mostly unavailable, more of his customers now creep along Bryant Avenue hoping for a vacant street space, or leave their cars standing with flashers while they run in for morning coffee.  But the bakery is still doing a brisk breakfast and lunch business, despite the parking situation.

"So far, people are adapting to it, think," Mayor Durkin said.