The Silver Line’s new Greensboro Metro Station is seen July 9 in Tysons Corner. (Jahi Chikwendiu/The Washington Post)

Metro is preparing for what experts say is its toughest challenge since opening 37 years ago: luring people onto the new Silver Line in Northern Virginia.

When the first phase of the 23-mile line opens next year, the gleaming new trains will no doubt revive the debate over whether mass-transit projects attract enough riders to make them worthwhile.

A recent Washington Post poll has found that it may be a tough argument to win.

The vast majority of Northern Virginians rely on cars to go where they need to go, and 70 percent of infrequent Metro riders say one reason they don’t ride the rails more often is that they just prefer driving.

A shrinking minority of Northern Virginians — 12 percent in the new poll — report taking Metrorail often, but riders and non-riders alike continue to give the system overwhelmingly positive marks. Half say they would ride the new Silver Line frequently or at least sometimes.

The Silver Line’s opening puts plenty at stake for a state that has been deeply conflicted over transportation priorities. Road advocates are pushing for new highways and wider arteries and have criticized the use of Dulles Toll Road revenue to help pay for construction of the new rail line.

If the Silver Line falls short of expectations, it could frustrate transit advocates’ efforts to further expand Metro and anger some riders of Metro’s Blue Line, who are already annoyed by service cuts made to accommodate the Silver Line.

“It’s been a very expensive gamble, but it’s one where the money has been spent and the commitment made, so it will be interesting to observe whether the forecasts will materialize,” said Jonathan Gifford, a transportation policy professor at George Mason University.

Car culture’s dominance

Nearly 20 years in the making, the Silver Line has been billed by officials as a transportation project that will spur development and job growth in the region for decades to come. The first part of the rail line is expected to open next year and go from Tysons Corner to Reston. The second phase, running to Dulles International Airport and farther into Loudoun County, is expected to open in 2018.

The new line gives Fairfax County a chance to cluster development of offices, retail space and housing closer to rail stations. With several stops in the median of Interstate 66, and little around them besides parking lots, some parts of the Fairfax stretch of the Orange Line have not stimulated as much development around Metro stations as planners had hoped.

But the car culture dominates the commute in Northern Virginia, according to The Post’s poll. Only 7 percent of commuters there take Metro; 85 percent drive to work. In Maryland, 75 percent of commuters drive, and in the District, fewer than half do.

As the region’s population has continued to grow, fewer say they are riding Metro regularly. Only 12 percent of Northern Virginians report riding Metro “very often” or “fairly often,” down from 19 percent in 2010 and 23 percent in 2005. By contrast, the poll finds Metro ridership steady among Marylanders and people living in the District.

Metro expects the new Silver Line to attract 25,000 new passenger trips on an average weekday in the first year. Of those, 15,000 will be people who may now drive, and 10,000 of those will be current Orange Line riders who will switch to the Silver Line, according to Metro. By comparison, Metro’s busiest line — the Red Line —carries roughly 288,000 passenger trips on an average weekday, according to an analysis of data from Metro. The Green Line is the lightest in passenger traffic, with an average of nearly 165,000 leaving from its stations, including transfer points.

How quickly drivers will change their behavior and sit on a subway seat is a bit of an unknown.

Dan Ostroff, who lives in Clarendon and works in Tysons, would seem to be an ideal customer for the new rail line, especially because his five-mile commute can take as long as 45 minutes on a bad day.

But Ostroff, an information-technology recruiter, said he isn’t sure that he will take the new rail line. He figures the fare would cost him about the same as the gas. And, he said, he’s grown to appreciate his time alone in the car.

“It’s my Dan time,” he said with a chuckle. But bad weather and worsening traffic conditions could persuade him otherwise, he said.

Michael Parbs said he may take it to his job in Tysons because the Wiehle Avenue stop is about a 10-minute drive from his Reston home. “It will be nice to have the option.”

‘Being built for the ages’

Transportation experts warn that it will probably take time for ridership on the Silver Line to ramp up and will depend in large part on how fast development along the corridor comes.

Alan Pisarski, a Northern Virginia resident and the author of “Commuting in America,” said the immediate impact will be small.

“I wouldn’t suggest the morning that it opens up that there’s going to be wonderful traffic relief in the region,” he said. “Rather, [people] are going to say, ‘Oh, did it open?’ The [Metro] system is very big, and changes — even billion-dollar changes — have limited impact.”

Ronald F. Kirby, director of transportation for the Metropolitan Washington Council of Governments, said the appeal of the rail line should not be underestimated. While today’s commuters may be set in their patterns, people new to the region — and those who just become tired of traffic — might give the rail line a try.

“The Silver Line is being built for the ages,” he said. “A lot of the ridership is going to be for people who are not here yet and jobs that are not here yet.

“You won’t find it heavily patronized when it first opens. It is designed to stimulate development around stations in Tysons and around Loudoun.”

That has worked before. Metro stations spurred development in Silver Spring’s aging downtown while reshaping Bethesda and the string of communities between Rosslyn and Ballston, making them national models for transit-oriented development.

A number of factors, including traffic conditions, fluctuating gas ­prices, the economy and the cost to ride Metro, will play a role in whether people will change their commuting habits to ride the new Silver Line, according to planners and transit experts.

“There’s a whole field of behavioral economics and incentives that affect people’s commuting behavior,” said Zachary M. Schrag, author of “The Great Society Subway,” a history of the Metro system. “Some of it is rational, some of it is emotional.”

In the late 1970s, for example, when the oil shock hit, “you had people who said, ‘I never thought about trying Metro’ until they saw gas prices skyrocket, and then they decided they’d rather sit on the train,” Schrag said. Ridership on the Red Line jumped

In coming months, Metro plans to hold open houses at the five new Silver Line stations.

The transit agency and Fairfax and Loudoun counties have also had dozens of public meetings to promote changes to area bus lines that will bring riders to the new stations.

Warren Fudge of Alexandria said he would probably take the Silver Line because it might open up more construction job opportunities to him in the Dulles region.

“I’d take that in a minute,” he said. “I’d really rather ride the subway than drive. It would be easier, and I could save a little bit of money.”

But among those who ride Metro sporadically in Northern Virginia, many say that stations are not close enough to their homes, that Metro doesn’t go where they want to go and that it takes too long to get from one place to another.

Fewer than half cite it as being too expensive or trains being too crowded as reasons they don’t ride.

Lori Aratani and Nicole Chavez contributed to this report. Clement is a survey research analyst with Capital Insight, the independent polling group of Washington Post Media. Capital Insight pollster Peyton M. Craighill contributed to this report.

An earlier version of this story incorrectly reported that the first phase of the Silver Line would open this year. It will open early next year.