The good news: Plans to build a bike/pedestrian path on the west span of the Bay Bridge—connecting San Francisco, Treasure Island and Oakland—are moving full-speed ahead.

The project will connect the East Bay to the city and peninsula, by bike and foot. When the bridge is done, you’ll be able to bike from San Francisco to Oakland in 45 minutes.

It will allow party people to get home in the wee hours. Commuters will blaze back and forth across the bay. And the tourists are going to love it: The north side of the bridge offers panoramic sunset views of San Francisco’s skyline.

The bad news: Full-speed ahead, for a mega-project of this size, cost and complexity, is rather glacial. Don’t expect to cycle across the bay any sooner than 2025.

Eight years to completion is the best-case scenario—and ten or more is likely. That was one takeaway from a public meeting Wednesday night put on by the Bay Area Toll Authority (BATA) and Arup, the engineering consulting firm designing the addition.

Arup civil engineers including principal engineer Rich Coffin and lead bridge designer Rafael Manzanarez gave short presentations with an emphasis on the main choices that need to be made about the new bike/pedestrian path. Will it run along the north side, south side, or above the existing bridge? Where will it touch down in San Francisco? Where will it touch down on Yerba Buena Island?

“We have the opportunity to create something really inspiring,” which could last 100 or 200 years if properly maintained, said Manzanarez. But, as he pointed out to a standing-room only crowd, although it’s only two miles from Rincon Point in San Francisco to Yerba Buena Island, bridging those two miles presents serious engineering challenges. In fact, adding a bike path to half of the Bay Bridge isn’t just a huge project. It’s three huge projects.

The first involves the downtown San Francisco touchdown, where a huge ramp must be integrated into a dense urban environment. There are six possible sites, three on the north side and three on the south. The three possible sites on the bridge’s more scenic north side include turning Essex Street into a bike/pedestrian area connected to the new Transbay Terminal, turning the Caltrans Paint Yard into a public space, or touching down at the Embarcadero. But the climb there is about 150 feet, requiring multiple loops, according to Sean Young, one of the Arup civil engineers who spent much of 2015 working on the San Francisco touchdown plan.

“From a designer’s perspective, we’re trying to come up with a simple path that can be used by all,” Young said. “But in some areas we recognize that there’s the opportunity to have an iconic structure, something that’s visually appealing, that people will talk about, that people will want to use.”

The most dramatic designs for the San Francisco touchdown include giant spiral ramps with as many as five loops.

The second part of the project involves the main span. The multi-lane bike and pedestrian path itself could be attached to the side of the Bay Bridge, or it could be suspended above the roadway.

The bike bridge will likely be about 15 feet wide, enough for two lanes of biking plus one for pedestrians. Pedestrian paths on the Golden Gate Bridge vary from 6 to 10 feet wide, in contrast.

Attaching the main span to the Bay Bridge is the “biggest challenge,” Manzanarez said, because it’s not easy to attach new steel to old steel. Old steel gets brittle. It can’t be welded. The old bridge must be partially taken apart and the new parts must be integrated using bolts and plates to replace existing rivets.

“It’s just surgical work,” Manzanarez said with a laugh. “It’s getting into somebody’s body.”

There’s also the issue of sinkage. Adding new weight will cause the bridge to be six to eight inches lower, potentially endangering very tall ships and angering the Coast Guard. To solve this problem, Manzanarez said that the suspension wires could be tightened, raising the roadway back up six to eight inches.

Finally, there’s the connection to Yerba Buena Island. Another massive on/off ramp must be built there, and safe bike paths must be found to reach Treasure Island. At one point, a bike tunnel may be necessary. Additionally, the Bike Bridge will provide an important connection for the growing population of Treasure Island, planners said.

At the meeting, the schedule for the project was presented: By the end of this year, there should be a preliminary design and cost estimate. Environmental studies will take about two years. The final design could be unveiled in about 2020. Construction is expected to take four to five years, and present its own set of difficulties. Most importantly, funding must be secured—a lot of funding.

Building the bike bridge will cost hundreds of million of dollars. On Wednesday, Peter Lee of BATA said the goal is somewhere south of $300 million. Some of that money could come from increasing existing tolls, he suggested, while other funds could come from state and federal transportation money.

At this point in the process, the engineers at BATA and Arup are keen to get public feedback and narrow down alternatives by the end of the year. The meeting was the first chance many members of the public had to see renderings of the alternate designs for the three parts of the project. It was also a chance to give feedback on the project directly to the people who are planning it. Meeting participants wrote comments on sticky notes or directly on renderings. They chatted with representatives from BATA, Arup and ICF International, the firm handling the environmental studies related to the bike bridge construction.

“We were asked to look at innovative ideas,” said Rich Coffin of Arup. “This is a major investment in infrastructure… an opportunity for a new architectural icon along the waterfront.”

One strategy to avoid sticker shock, said John Ciccarelli of Bicycle Solutions, a consulting service, is to present a core design with optional add-ons. He likened it to a main course with optional side dishes. Politicians and planners could pick and choose features based on priorities and budget constraints, he said.

Once the bike connection across the bay is completed, Ciccarelli said, it could prove a boon to Oakland, acting as a new transport hub.

“If this becomes cool, development could respond,” Ciccarelli said. “Imagine Mandela Parkway starting to dot up with residences. Wouldn’t that be cool?”