When the $13 million Monroe Avenue Bridge over the railroad yards in Alexandria was opened partly on Tuesday, it was the beginning of the end of a 13-year wait by area motorists.

The bridge, which has four lanes and one turning lane, carries about 50,000 vehicles a day that travel to and from Washington along Rte. 1. It is the third heaviest traveled north-south route in Alexandria, surpassed only by I-395 and Washington Street.

The old Monroe Avenue Bridge, next to the new bridge, has long been one of the worst bottlenecks for commuters in Northern Virginia. It now will be dismantled, a piecemeal job that will take three months. Sections of the bridge will be cut up and lifted out by crane so rail lines beneath the bridge are not blocked.

"This bridge has not been built to solve a traffic problem; this bridge was built to replace a 54-year-old bridge that was structurally inadequate to handle the traffic," said Dayton Cook, Alexandria's director of transportation and environmental services.

Cook said that in 1974 the city spent $500,000 to reinforce the old bridge so it could handle heavy trucks until a new bridge was built.

"Eventually we would have had to close the {old} bridge to trucks and it's a major fire truck route," Cook said. "Can you imagine no trucks being able to get from the Del Ray area to downtown Alexandria? That's why this bridge was built."

The construction of some ramps leading to the bridge will continue for at least another four months. In the meantime, temporary routes will be used to carry all traffic onto the bridge.

Access roads that will be built include one that will carry traffic from Slaters Lane south underneath the new bridge to connect with the southbound lanes of Rte. 1.

The bridge will have traffic lights at each end. There have been only grade changes to the west end of the bridge where it intersects with Monroe Avenue, just before Rte. 1 turns sharply north and continues into Crystal City.

Cook said many commuters, especially people who live in Fairfax and Prince William counties and the Oxon Hill and Fort Washington areas, have awaited eagerly the opening of high occupancy vehicle lanes on Rte. 1 in Alexandria. Those HOV lanes are scheduled to open Jan. 4, and they will run from the Capital Beltway to the Monroe Avenue Bridge. Lanes will be added to a short portion of Rte. 1 just south of the bridge so that the route will be six lanes for the entire HOV stretch.

Two roads that previously crossed the triangle bounded by Powhatan Street and Slaters and Bashford lanes just to the east of the bridge have been closed permanently. Cook said that the city, which owns that triangle of land, plans to sell the land after the project is completed.

He said the city soon will begin widening Rte. 1 from Reed Avenue north to 25th Street from four to six lanes. There are no plans to widen the four-lane section of Rte. 1 from the Monroe Avenue Bridge to Reed Avenue, Cook said.

Part of the $13 million bridge project will help alleviate future traffic projected by city planners from what is now an undeveloped 38-acre site just north of Slaters Lane. This year two developers proposed to build there the controversial $500 million, 16-building Potomac Greens complex, which Old Town residents and the City Council opposed, saying it was too large and would generate too much traffic.

"Some day there's going to be something on the Potomac Greens site and this {bridge} project will serve the site by allowing 500 more cars per hour to enter and exit the Potomac Greens site from Slaters Lane and Rte. 1," Cook said.

In 1972, the city selected a contractor to build a twin bridge span for Rte. 1 to cross the rail yards, but neighbors opposed it, and eventually the project was defeated in the courts on environmental grounds. That span would have been six lanes with no traffic lights. Cook strongly favored it.

"This bridge is the best we'll ever get for this spot, so it's always going to be somewhat of a bottleneck," he said.