Passengers wait on the platform before boarding a train at the U Street Metro station in the District. (Pablo Martinez Monsivais/AP)

The answers could be worth millions — even billions — of dollars: Where have Metro’s riders gone, and will they ever come back?

In the most recent quarter alone, the beleaguered transit agency reported an 11 percent drop in Metrorail ridership. In the last fiscal year, Metrorail riders took 15 million fewer trips than the previous year.

Those numbers are likely to get worse before they get better. The 11 percent drop came just as Metro was ramping up its SafeTrack program, an unprecedented attempt to catch up on much-needed maintenance. The program requires full or partial shutdowns of portions of track throughout the system, forcing most riders to find alternative transportation — something Metro has actually encouraged.

Where are those riders going? Definitive answers are hard to come by, and statistics, in some cases, are contradictory. Everyone, it seems, has a theory.

[Metrorail ridership plunges 11 percent]

Jack Evans, the Metro board chairman, is convinced that the transit agency’s reliability issues, coupled with the long-term disruptions of the SafeTrack program, which began in June, have driven more people back into their cars.

“I can tell by the congestion on the streets — [downtown Washington] is a wreck,” Evans said recently. “There are probably some smaller things that people are doing — working from home, that’s a thing — but my view is with the congestion on the roads these days, people are driving.”

Federal statistics that indicate more Americans are driving seem to bolster that view.

[More Americans are driving and that might not be a good thing. ]

Transportation experts say it’s not that simple, particularly given that Washington-area commuters have a plethora of options for getting around. While it’s impossible to say definitively where those missing Metro riders have gone, recent data about commuting habits offers clues, they say.

A new survey by the region’s Transportation Planning Board, for example, found that the number of solo drivers has declined over the past six years — 61 percent of commuters reported driving alone, down from 66 percent in 2013.

Perhaps that shouldn’t be surprising.

Getting around the region is no longer a simple choice between driving and taking Metro, said Nicholas Ramfos, director of transportation operations programs at the Metropolitan Washington Council of Governments. Commuters, he said, now can use bike sharing, ride sharing or car sharing, or can even walk. And while many commuters tend to be creatures of habit — often taking the same routes or using the same modes, Metro’s troubles have forced many to explore other options.

If there is any overall trend that the planning board has identified since it began surveying people in 2001 through its Commuter Connections program, Ramfos said, it is that people around the region are more flexible in when, how and if they are going to commute.

Sam Zimbabwe, acting chief project delivery officer at the D.C. Department of Transportation, agreed.

“It used to be that someone would wake up in the morning and know exactly what train they were going to catch,” he said. “Now people wake up and ask: ‘Do I want to bike? Take an Uber? Telework?’ People are making their choices on a day-to-day basis.”

Melissa Dentch, 30, is an example of someone who is no longer wedded to a single way of getting around. She lives 2 1/2 blocks from a Metro station and used to use the transit system to get to her job at Georgetown University almost exclusively. Metro’s troubles prompted her to find other ways.

“I kind of feel like I’m Metro’s worst nightmare,” she said, explaining how she gets around town. “I own my own car, I have a Car2Go membership, I own a bike and I do bike share. There are so many options for the price that Metro’s charging . . . unless things improve, people are just going to find other options.”

The planning board’s survey also found another possible reason that fewer people may be riding Metro: They no longer commute every day. Significantly more Washington-area workers are teleworking, with nearly one-third working from home at least one day a week or more. That’s up from 27 percent three years ago. Researchers say that’s a gain of about 200,000 teleworkers.

Ramfos said that if large numbers of workers are commuting four days a week instead of five, it could certainly have an impact on Metro’s bottom line. And if they no longer use Metro to get to work, they might have stopped using Metro for non-work trips, too.

Ramfos has another theory about why Metro riders may be disappearing: demographics. Baby boomers are retiring in growing numbers — a trend that has hit the federal government particularly hard. The younger workers, he said, may be less wedded to the Metro system. About 32 percent of Metrorail riders are federal workers.

[A wave of retirements is sweeping through the federal workforce]

The survey also showed a slight uptick in the number of people biking to work — 3 percent vs. 2 percent the previous year.

In the District, officials have seen spikes in vehicle traffic that probably are linked to Metro’s SafeTrack program, but they don’t think that all of those former Metro riders have simply returned to their cars.

Zimbabwe, of the D.C. Transportation Department, said that recent Census Bureau data indicated a significant increase in the number of people who said they walk to work — roughly 8,000 more.

In Arlington, bike counts have increased along the Custis and Mount Vernon trails — the most significant of which came during Metro’s first surge, which affected the Orange and Silver lines between the East Falls Church and Ballston stations.

And, more commuters appear to be ride-sharing. When officials in May announced a new arrangement that allowed Car2Go users to travel between Arlington and the District, the number of trips more than doubled, Arlington County spokesman Eric Balliet said. The shift also overlapped with SafeTrack work, so Balliet said it’s possible that the frequency of trips was influenced by track work.

Commuter rail also has benefited. Officials at Virginia Railway Express, which has routes that parallel the Metro system, said ridership is up from 2015 with spikes that coincide with SafeTrack work. Even so, spokesman Joe Swartz said, he thinks it will take a year to see whether the gains stick.

Winning riders back is critical to Metro’s financial health. A consultant’s report presented to the board in July warned that if current trends continue, the agency will face a budget shortfall of $1.1 billion by 2020.

Metro officials say they aren’t worried — at least for now. That view might be bolstered by research from the University of Maryland’s National Transportation Center and George Mason University, which is studying the effects of SafeTrack on commuter habits. One of the most surprising finding so far?

During the first surges, “People didn’t really change very much,” said Chenfeng Xiong, a research scientist with U-Md.’s National Transportation Center. “A lot wanted to stay with Metro even though they knew about the potential service delays.”

Metro General Manager Paul J. Wiedefeld is confident that riders who have left the system will return.

“I am very confident that the ridership will come back up,” he said. “That [traffic congestion] is here, and it’s not just a pure function of what’s going on with Metro. I think for a very large portion of the people [Metro] does work well.”

As for other players in the transportation market, Wiedefeld doesn’t view them as competition.

“When you think of your trip, you may do all kinds of things — you may drive, you may walk, you may bike, you may take transit or an Uber,” he said. “That’s not a bad thing; it’s all part of the way we live, so if you make [Metro] more reliable, then we’ll be in play more.”

And Wiedefeld figures he has another ace in his pocket.

“Let the price of fuel go up to $5, and we’ll see.”

A previous version of this article said Metro reported an 11 percent drop in ridership in the most recent quarter, and that in the last fiscal year, Metro riders took 20 million fewer trips than the previous year. Metrorail reported the 11 percent drop for the quarter, and Metrorail reported about 15 million fewer trips for the most recent fiscal year. The previous version also said about 36 percent of Metro riders are federal workers; about 32 percent of Metrorail riders are federal workers.