For nearly 50 years, Northern Virginia commuters who work in the District have flocked to Old Keene Mill Road in Springfield and picked up complete strangers for the ride into the city.

This organized hitchhiking — or “slugging” — allows drivers to use high-occupancy vehicle (HOV) lanes and cut 30 minutes or more off their commute through crawling traffic. And the riders, many of them federal workers, policy wonks and military personnel, get a free ride.

But Bob’s Slug Line, the region’s oldest, is now threatened.

[The ‘original ride-sharing’ system: Slugging.]

Fairfax County is planning to build a $63.8-million transit center at the site, which could temporarily displace the hundreds of commuters who take part in the nontraditional carpooling from the park-and-ride lot off the Springfield Interchange — better known as the Mixing Bowl — where interstates 495, 395 and 95 collide.

“Two years of construction will hurt. It will impact 1,000 riders,” said Kalai Kandasamy, a slug line organizer. “Creating a slug line is very hard, while disrupting a slug line is very easy.”

County officials say the multi-use garage planned for the nearly three-acre site will expand parking options for sluggers, bus riders and other commuters in the area. Limited parking for sluggers has been a problem for decades.

The project, expected to be completed in 2021, will have a garage with 1,100 parking spots, a hub with seven bus bays and a secure room for parking up to 90 bicycles.

“We want to provide any commuter with options to get to where they need to go,” said Michael Guarino, the project’s manager with the Fairfax County Department of Transportation.

[Next on Virginia’s extensive toll-road system: Interstate 395]

But commuters are worried Bob’s Slug Line won’t survive the construction. They already encounter a limited pool of public parking in the area that makes it difficult for those seeking rides each morning, and the county hasn’t found a location to relocate the slug line once construction starts next year. Officials, however, dismiss commuters’ perception that the county doesn’t want to keep the slug lines functional during construction. They say they’re in conversations with property owners nearby to find a spot to relocate Bob’s.

“We know it is going to be a challenge,” Guarino said. “We want to be as close to the site as possible so that it still works for everybody using the slug lines.”

Bob’s — the name sluggers preserved for the site even after the Bob’s Big Boy for which it was named was gone — has been through displacements, parking reductions and even county regulations since it was created in 1974, shortly after HOV lanes opened on Interstate 395.

Before it became slugging, the informal system was known as Springfield Underground, the Commuter Pickup and Ride Share. Some theories say it began with one driver who wanted to use the HOV lanes asking riders waiting at a bus stop if they wanted a ride. Others say it was born out of frustration with gridlock and flourished exclusively through word-of-mouth.

By 1980, the Springfield slug lines were so popular that they were taking a toll on bus ridership and contributed to a deficit of $250,000 a year for the county transit system, according to news reports. By 1991, Fairfax County was installing signs prohibiting stopping, and officers were issuing tickets to motorists following complaints about traffic near the slug lines.

County officials said then that they were in favor of the informal arrangements as long as the operations ran unobtrusively. Today, they say they want to facilitate slugging.

In 1990, the old Springfield Cinema movie house site used by sluggers was redeveloped into a 31,000-square-foot Circuit City, displacing the sluggers. It is unclear where they went then, but news reports suggest many continued to park at Circuit City or nearby parking lots.

When the store closed, the county purchased the property with the intention of building a parking garage, officials said. As an interim, it created the existing park-and-ride lot, providing 280 parking spaces. Fairfax also spends about $100,000 a year to lease another 400 parking spots from nearby businesses and nonprofit organizations.

Leasing parking hasn’t been easy in recent years as the area continues to grow. County officials anticipate it will get more challenging in an area ripe for development.

Guarino said when lease terms are up, the county negotiates to renew but can’t guarantee those spaces will be available in the long run. In 2009, a lease fell through and so went 75 spots.

“This project is a way to consolidate those spaces on a county-owned property that we have control of,” he said.

If the slug system is to be preserved, officials and even some commuters agree, more parking will be needed, and the county garage may be the answer.

A recent morning found a crowd of well-groomed D.C.-bound workers on two lines, each haggling with passing motorists about their destinations.

“L’Enfant!”

“14th and K.”

“Union Station!”

By 8 a.m., many of the 680 available parking spots were filled, and the slug lines were as robust as they were an hour earlier. Cars pulled in to pick up passengers, slowing traffic from Old Keene Mill Road.

“You cannot beat a flexible carpool solution,” said Edward Shipley, 43, a 20-year slugger who was commuting from Bob’s to Seventh and D Streets SW.

But Shipley said there are too many unanswered questions about the transit-center project. Many commuters still don’t know about it. Others worry the construction will move the slug lines too far away or further reduce parking, And once the new garage opens, some are concerned about parking fees or worse, being pushed to take transit.

“Most of us in Springfield are not interested in getting on a bus to be dropped off at the Pentagon and having to jump on the Metro to then continue on to our jobs,” Shipley said. “Slugging is a great solution to not having to meet the bus schedules.”

Transportation officials say the project, which has been in the county’s comprehensive plans for many years, will create a central location for commuters to catch as many as 10 Fairfax Connector routes serving the area. It is viewed as another strategy to improve bus service and help the county’s bus system rebound from declining ridership. The Springfield transit center would be one of a handful of bus-only hubs in the county, but it will be unique, officials say, in that it will also serve sluggers, bicyclists and other non-bus commuters.

[Think rush-hour, peak-direction tolls on I-66 are bad? How about tolling drivers going both directions?]

It will include boarding areas for sluggers, with space for up to two slug lines and queuing capacity for 18 cars in one line and 15 in the other. The slugging area was redesigned to add space for more vehicles after some commuters complained that the project was taking slugging space away for the bus service.

“Eventually it will be great,” said John Venable, who pulled up shortly after 7 a.m. Thursday to pick up two passengers headed to Union Station. “But there are going to be growing pains.”

Pat Sweeney, a federal worker who has picked up passengers at Bob’s since 1990, said she’s tried many commuting options, but the impromptu carpooling system provides the most effective and fastest way to get to work.

Anthony Allen jumped into Sweeney’s car at 7:30 a.m., and the two waited for a third passenger headed to L’Enfant Plaza. A few other cars were headed there too, so it took about five minutes to find the third rider they needed to meet the HOV requirement.

“It’s totally luck,” Sweeney said. But there’s never a lack of passengers. Sometimes it takes them longer to get to the slug lines because there’s no parking. Allen himself has to take a bus to the parking lot.

“The new garage will help,” Allen said. “It is also timely. This lot is not sustainable in the next five years. There’s just too much demand.”