We’re getting the first glimpses of what a Bay Bridge bike path from Treasure Island to San Francisco would look like — and the ideas range from the spectacular to the surreal.

But as far out as some of the designs might be, one thing is clear: What was once viewed in transit circles as pure fantasy dreamed up by the bike-riding crowd could become very real.

In fact, if it gets the blessing of the Legislature, a sky path could go before the voters in 2018 as part of a plan to boost tolls by $1 on the seven state-owned bridges in the Bay Area — that is, every span across the bay except the Golden Gate.

The cost of the 2.9-mile bike and pedestrian pathway is likely to run into the hundreds of millions of dollars — the exact number is anyone’s guess. But what it might look like is coming into sharper focus, thanks to new mockups produced as part of a $10 million engineering study under way by the Metropolitan Transportation Commission.

The various alternatives are being circulated as part of a presentation to interest groups that back a pathway.

“We really think it’s going to happen within the next 10 years,” said Renee Rivera, executive director of Bike East Bay, a group that has been pushing for a Bay Bridge bike path for more than two decades. “I’d put my money on it.”

Adding a bike and pedestrian path to the 80-year-old bridge poses a number of engineering challenges. One of the biggest would be how to get bicyclists and pedestrians onto and off of a bridge that enters the city some 150 feet above ground.

Three main choices have emerged:

•A sweeping descent along the Fremont Street exit ramp on the north side of the bridge to Essex Street. The path would then turn quickly westbound onto Harrison Street.

•A four-story-tall series of circular loops descending from the south side of the bridge to what is now a Caltrans yard between Harrison and Bryant streets.

•A six-story-tall series of loops that would descend to the waterfront from a pathway that would be suspended over the bridge’s upper deck.

“The ideas were purposely broad and purposely meant to show you everything, which is why some of them — on their face — are more possible than others,” said Randy Rentschler, a spokesman for the Metropolitan Transportation Commission.

Rivera said she favors the Essex Street landing as the most practical and probably the least expensive alternative.

The options for the pathway itself are no less challenging. It could be suspended over the road, as in the six-story-tall loop descent plan, or it could be attached to the side of the bridge. The big unknown is whether, under either scheme, the weight would cause the bridge to sag so much that mega-ships heading for the Port of Oakland might not clear it.

Barring a federal waiver of Americans with Disabilities Act requirements, any plan will include a 150-foot tall elevator to make the path disabled-accessible, Rentschler said.

If bike advocates have their way, the path could be open for walkers and riders sometime in the 2020s. It would be the completion of a drive that picked up considerable steam when Caltrans included a pathway on the new eastern span of the bridge, which opened in 2013.

Proponents are banking on thousands of commuters from the East Bay and Treasure Island — where 20,000 people eventually are expected to live in new high-rise towers — to justify the expense of building a western-span path.

“The bridge really has no way to add capacity, except allowing people to bike,” Rivera said.

The thinking at the Metropolitan Transportation Commission is that if the price tag can be kept at under $300 million — something that a team of private consultants will be analyzing in the next year — the project has a real shot of being funded as part of a much bigger regional transportation measure. That measure would raise peak-hour Bay Bridge tolls to $7, and to $6 on every other state-owned bridge.

The bridge pathway will also be competing for money with a long wish list of other big-ticket transportation projects — everything from a second BART Transbay Tube to bringing high-speed rail into the new Transbay Transit Center. And whatever goes on the ballot will have to be authorized by the state Legislature.

In other words, said Rentschler, “it’s a big puzzle, and the politics has to work itself out.”

Schaaf on love: As we reported Monday, Oakland Mayor Libby Schaaf reacted coolly to Councilwoman Lynette Gibson McElhaney’s call to post the motto “Love Life” on all the city’s welcome signs as a way to combat its image as the state’s murder capital.

“My love life is fine,” Schaaf quipped when we asked about the proposal last week.

The slogan idea has long been pushed by Donald Lacy, whose teenage daughter LoEshé Adanma Lacy was shot to death in Oakland in 1997. LoEshé means “love life” in a Nigerian language.

Well, now Schaaf says that when we first talked with her, “I did not know about the background story or context, and I gave an off-the-cuff response that I now see was horribly and inexcusably insensitive.”

But while she supports the Love Life movement and the sentiment behind it, she said, “As my own regrettable gaffe shows, it’s nearly impossible to adopt a motto for the city of Oakland that, without story or context, captures the essence of Oakland and is universally understood and appreciated.”

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross