Commuters idled in traffic jams, planners spent countless hours on studies and politicians debated for more than a decade as road crews patched and paved a crumbling Frederick Douglass Memorial Bridge.

Now, a replacement span is about to open this week.

The city is transitioning from the 71-year-old bridge in Southeast Washington to a freshly built, 1,445-foot-long structure with a community party — opening to pedestrians and bikes for a one-day preview Monday — before welcoming cars for good by the end of the week. The replacement is the city’s largest infrastructure project in history, valued at $480 million, and it is viewed as a critical step toward the transformation of the shores of the Anacostia River.

“It’s great to see something come to fruition,” said Sabreena Geddie, a lifelong D.C. resident who was one of hundreds of people to make an inaugural crossing.

The bridge is not finished. Vises hold up 2-by-4s as makeshift barriers on concrete lookout areas, and paving and connections aren’t complete on wide pedestrian pathways. But Monday’s bridge celebration — starting with a 5K run and walk led by Mayor Muriel E. Bowser — offered expansive views and the chance for many to consider a changing city.

Bowser (D) called the bridge a symbol of “the best of our city’s past and future,” and said it is fitting “that we come together to celebrate a bridge that will connect our city and pay tribute to a great Washingtonian.” On Tuesday, she will be joined by other leaders, as well as descendants of Frederick Douglass, for a ceremonial ribbon-cutting.

The new bridge will open to outbound vehicle traffic Friday and to inbound traffic Saturday, weather permitting, transportation officials said.

Runners, walkers and stroller-pushers began crossing the span about 9 a.m. Monday as an American flag waved from one of its towering white arches.

“We’ve been watching it for years while it’s been under construction and it’s beautiful — and D.C. deserves it,” said Marsha Lea, a retired landscape architect from Capitol Hill who said she was moved by the idea of a new connection tying the city together.

Melodye Robinson, a health-care worker from Southern Maryland, was mostly hoping for an easier drive. She said traffic patterns on the old bridge could be tough to navigate.

“Oh boy, you better pay attention,” Robinson said. “It was convoluted. Hopefully it will just be easier.”

The opening of the distinct structure with parallel arcs marks a milestone in the vision to turn the South Capitol Street Corridor into the grand boulevard that Pierre L’Enfant envisioned in his original plan for the nation’s capital. L’Enfant identified the corridor as a symbolic gateway into the District’s monumental core, one that has not been realized as the road maintained a freeway-type configuration that lacked adequate pedestrian and cycling facilities.

Officials say the new bridge visually and aesthetically supports that urban-boulevard vision by improving the view toward the U.S. Capitol and monumental Washington from the Anacostia.

“The arches, and the span of the arches, it just brings more life to the bridge,” said Nettie Stewart, of Kensington, Md., who draws bridges using computer design software at her day job. “It’s not just a piece of concrete.”

When completed, the stretch just outside the bridge will feature open green spaces and seamless connections to Anacostia River parks and trails. By spring 2022, there will be new traffic ovals at each end of the bridge and sections of South Capitol Street will be re-created as a scenic boulevard with landscaping on both sides.

The project also includes the reconstruction of South Capitol Street’s interchange with the Suitland Parkway and Interstate 295.

The new bridge was designed to relieve congestion on one of the busiest gateways into downtown and to increase pedestrian and auto safety, while increasing access to the river.

It includes six general travel lanes — one more than the existing bridge. It also has 18-foot multiuse paths on both sides for two-way bike traffic separated from pedestrians, although work continues on those sections, as it does on areas of each side of the bridge that will provide space for community activities.

On the old bridge, pedestrians and bicyclists are squeezed into a roughly five-foot concrete sidewalk along each side.

When the bridge opened in 1950 — simply referred to as the South Capitol Street Bridge — it became a direct link between downtown Washington and neighborhoods east of the river, as well as communities in Prince George’s County. Before the pandemic, up to 70,000 daily commuters traveled through the corridor.

The bridge was dedicated to abolitionist Frederick Douglass in 1965. Douglass, born into slavery on Maryland’s Eastern Shore in 1818, escaped as a young man and became a leading voice in the abolitionist movement. He moved to Washington in 1872.

“Frederick Douglass would be smiling in his grave right now. I’m smiling for him,” said James Lovell, a musician from Gainesville, Va., who originally is from Belize. He said he hopes someday to share his indigenous Garifuna musical traditions in one of the gathering spaces that are part of the bridge’s design.

D.C. transportation director Everett Lott said the city expects that once the work is completed, the corridor will be a magnet to draw people together.

“It is going to attract a lot of people to come out and have another place to be able to gather safely and really enjoy nature and enjoy the views,” he said. “We have so many national treasures and iconic monuments throughout the District, and this is going to be another one of those.”

The new bridge’s construction, which started in summer 2017, followed more than a decade of planning to address the deterioration on the old bridge. Corrosion has eaten holes through thick steel beams that were placed seven decades ago. It has been deemed structurally deficient for years.

The old bridge is still safe for travel, thanks in large part to a rehabilitation project that shut down the span during the summer of 2007. But as with thousands of bridges across the country, it was falling apart faster than repairs could be made, engineers say.

On Monday, Erin Wiley, who works in government relations and lives in the Navy Yard neighborhood, crossed the old bridge for the first time — by mistake. She and a friend thought they were headed for the new span.

“There are big chunks of this one that are not there anymore. I’d feel safer on the new one,” she said. Wiley, who is afraid of heights, said the old bridge’s middle section — which once could be shifted to allow ships to pass and allows for stark views of the water below — was a source of angst.

Indeed, one of the bridge’s aging pedestrian walkways is missing large chucks of concrete, revealing rusting steel below.

Del. Eleanor Holmes Norton (D-D.C.), who helped secure more than $200 million in federal funding for the new bridge, said the project is not only the city’s largest she could recall, but also one that has taken the longest to build.

“I was able to get earmarks [by] screaming that one of these days we were all going to be embarrassed because the South Capitol [Street] bridge was going to come tumbling down,” she recalled in a 2017 interview.

More recently, she called the new bridge one of the most iconic gateways into the seat of the federal government and “a shining example of federal infrastructure dollars at work.”

City leaders tout the new structure as more in line with development in the area, which has transformed from industrial and military uses into thriving mixed-use communities. Growth has peaked around Nationals Park and Navy Yard at the foot of the bridge. And city leaders say they expect it will help spur economic development in Ward 8.

“I love how it connects the two areas. For so long, it seems like the neighborhoods east of the river haven’t had that kind of access or resources,” said Tobi Pratt, a federal worker from Bladensburg, Md., who crossed the bridge with her son, 7-year-old George, who was carrying a red, white and blue balloon fish from the community celebration.

Crews completed the bridge using more than 15,500 cubic yards of concrete, 700 tons of structural steel and 12,000 linear feet of steel piles. More than 150 workers have been at the site almost daily since construction began in summer 2017.

DDOT spokeswoman Lauren Stephens said the city will begin to dismantle the old bridge as soon as traffic is moved to the new bridge. This work is expected to continue through spring, when a new Suitland Parkway/I-295 interchange should be complete and the old bridge dismantled.