For the past decade, the transit center that will replace San Francisco’s Transbay Terminal has been the subject of grand plans and political controversies, struggles to stay on schedule and squabbles over costs.

Next weekend, all that changes.

On Aug. 12, transbay bus service will begin at the $2.16 billion Transbay Transit Center, which stretches nearly three blocks between Beale and Second streets, just south of Mission Street. East Bay commuters entering the city by bus will travel traffic-free above downtown streets from the Bay Bridge into an elevated concourse.

The new station should also attract Bay Area residents and visitors who never ride a bus. For them, the lure is a 5.4-acre park that fills the roof above the concourse: Eight distinct gardens ring a long, oval path. Within the park is everything from a picnic meadow and a children’s play area to a two-story restaurant with a terrace 80 feet in the air.

There’s plenty else up in the air, figuratively speaking, including such questions as whether commuter trains will ever find their way to the transit center, and whether the rooftop park will be marred by the open drug use and disturbing behavior that now make some downtown blocks seem unsafe. Or, as planners and boosters hope, it will become the busy centerpiece of the new high-rise district around it.

“From this perspective you really get the layering,” said Adam Greenspan, standing in the middle of a lush meadow, steep towers on all sides. “The green mound, the glass mound, the lawn, all rolling over the architecture.”

Greenspan is a design partner at PWPLA, the landscape architecture firm responsible for the transit center’s rooftop park. On the west, the meadow blends into a hillock topped by Australian pines. On the east, it meets a children’s play nook.

The firm is part of the design team led by Pelli Clarke Pelli Architects. which was selected in 2007 by the Transbay Joint Powers Authority — a public agency created in 2002 to replace the terminal that opened in 1939 as part of the Bay Bridge project. When demolition of the little-loved original began late in 2010, the target date to complete the first phase of the new complex was October 2017, with commuter trains and high-speed rail arriving at an underground concourse two years later. Budget for the overall project: $4.2 billion.

Now, the earliest date for train service is 2029. The budget is at least $4 billion for the rail phase alone. And neither number is carved in stone, since most of the second-phase financing hasn’t been pinned down.

There have been other changes.

Instead of paying tribute to some politician, the facility now officially bears the name Salesforce Transit Center, with Salesforce Park on top. That’s the trade-off for a 25-year, $110 million sponsorship deal with the tech firm, which is headquartered next door.

The rooftop park, though, meets the early promise of a verdant and vivid world unto itself.

“This isn’t just a lawn dotted with trees,” said Peter Walker, founder of PWPLA and a landscape architecture legend whose Bay Area work dates back to the 1960s. “This is a roof garden, and it has its own story to tell. We want to connect on an intellectual level as well as an aesthetic one.”

Now Playing:

That approach might not please visitors in search of nothing more than, well, a lawn dotted with trees. But the goal is a kaleidoscope of overlapping environments — from the drought-tolerant succulents at one end of the park to the water-friendly rushes and river birches at the other. In between are terrains including an Australian garden and a “prehistoric garden” and an oak meadow, plus the large central plaza and a grass amphitheater that can hold an estimated 800 people.

Downstairs, the mood inside is airy rather than lush.

The Grand Hall, entered from a plaza at Mission and Fremont streets, has 32-foot-high glass walls and a sparkling terrazzo floor embedded with mosaics of flowers and birds — one of four large works of public art, this one by Julie Chang. Beneath a large skylight there’s an escalator that takes you on a 52-second journey to where buses will loop slowly around a central island that offers waiting passengers fragmented views of nearby buildings through the outside wall’s lacy metal panels.

All this is a contrast to the center’s hulking concrete predecessor, as well as the informal, open-air temporary transit terminal at Howard and Beale streets. The aim is to give bus travel a cachet — no easy feat in today’s America.

While the transit center and its rooftop park are an unusual pairing, they’ll be managed in a way that’s in sync with the latest set of high-profile city parks. As much space as possible is programmed, with events and offerings designed to attract different groups throughout the day.

The model is Bryant Park, which shares a block of midtown Manhattan with the New York Public Library. Long a haven for drug dealers and petty criminals, the space was rejuvenated in the 1990s — with not just physical upgrades but also an array of mostly free enticements, from juggling classes to a well-stocked outdoor “reading room.”

From the start, the person in charge has been Daniel Biederman. He since has branched out to other cities through his own firm, Biederman Redevelopment Ventures, and it is part of the transit center’s management team.

Visitors the first week can expect craft booths and exercise classes, according to Ashley Langworthy, director of Biederman’s local office. There will be games to check out and free music, Toddler Tuesdays and a lunchtime writing workshop on Wednesdays.

While some zones will be heavily programmed, others will be more tranquil.

“The central plaza is the hub of activity,” Langworthy said. “But it’s crucial to have areas where people feel they’re in a natural setting, that they can sit by themselves.”

An empty bus does a training run on the transit deck at the Transbay Terminal in San Francisco, Calif., on Tuesday, July 31, 2018. The new transportation hub will open on August 11. An empty bus does a training run on the transit deck at the Transbay Terminal in San Francisco, Calif., on Tuesday, July 31, 2018. The new transportation hub will open on August 11. Photo: Photos By Carlos Avila Gonzalez / The Chronicle Photo: Photos By Carlos Avila Gonzalez / The Chronicle Image 1 of / 10 Caption Close San Francisco’s imposing transit center ready to roll at last 1 / 10 Back to Gallery

The stand-alone restaurant on the west side of the park will be vacant in coming months, as will the retail space in the floors below — ripple effects of the delay in selecting a management team while the joint powers authority wrestled with a budget that climbed from $1.6 billion past $2 billion. Instead, kiosks inside the center’s Grand Hall will offer shoeshines and flowers, coffee and pastry. As many as a dozen food trucks will be deployed each weekday on Natoma Street, and there will be a public market on Wednesdays at 3 p.m.

The programming has a defensive aspect, as well. Without it, there are fears the transit center might become a void that attracts troublemakers or drug users looking for a place to shoot up.

Part of this is a reaction to the conditions in other downtown settings, such as United Nations Plaza and the Mid-Market area. It’s also a reflection of the fact that in later years, the old terminal became known as a spot where homeless people loitered by the dozens while commuters hurried in and out.

Now Playing:

“Anyone who remembers the old terminal has anxiety,” said Andrew Robinson, executive director of the East Cut Community Benefit District, a neighborhood group funded by property owners.

This time around, multiple layers of security will be present at the transit center, which will be open from 5 a.m. until 1 a.m. daily. The park will close at 9 p.m. during the summer and fall and 8 p.m. the rest of the year.

There will be uniformed security guards as well as “ambassadors” clad in orange polo shirts. San Francisco police will walk the beat around the clock, inside and out.

The aim is to create a secure atmosphere, but not an oppressive one. Think Yerba Buena Gardens, the popular public space two blocks to the west where you see people of all types lingering in the grassy bowl. On a sunny afternoon, conventioneers and nearby workers relax alongside other visitors who look as if they have nowhere else to go.

“We’ll be working to strike a balance,” said Sidonie Sansom, the authority’s chief security officer. “We want people to feel comfortable here, welcome. But you also have to have rules and regulations to follow.”

Ultimately, the inauguration of bus service at the transit center is just one part of a much larger journey — a journey that already has had missteps and surely will have more.

More Information Online extra John King will talk about the Transbay Transit Center and park at noon Monday on Facebook Live: www.facebook.com/sfchronicle Transit center open house Buses won’t yet be running, but the new Transbay Transit Center opens its doors to the public on Saturday. The main activity on day one is a block party that kicks off at noon with a performance by the West Grand Brass Band in the rooftop park’s central plaza. The scheduled events conclude with a sampler of Shakespearean scenes by the African American Shakespeare Co. at 4 p.m. in the western amphitheater. Throughout the afternoon there will be exercise classes and kids’ activities, including a petting zoo. The entire building is open for exploration, and from noon until 4 p.m. people can stroll the bus bridge that connects the transit center to the Bay Bridge — a one-time-only opportunity. For more information: http://salesforcetransitcenter.com

But consider what has been accomplished. A major work of 21st century infrastructure has not only been summoned into existence, it has helped summon up a neighborhood around it. The notion of a larger public — public transit and public spaces and large-scale public investment — is being celebrated, not scorned.

With luck, the completion of the transit center’s first phase will be a prod to future action. In this scenario, city and regional officials will work to make things happen rather than embark on yet more rounds of studies. The developers who have erected towers nearby will use their connections to make the case for finishing the job, so that the transit center becomes the multimodal destination promised in their marketing hype.

But all that lies in the future.

In the meantime? There are buses to catch and a park to explore.

John King is The San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: @johnkingsfchron