OmniRide, a commute bus that runs to Metro stops, picks up a rider on Center Street in old town Manassas. ( Margaret Thomas/The Post)

Falling revenue from a regional gas tax earmarked for transportation, including Metro, has Northern Virginia governments scrambling for cash to maintain their transit systems. And in the most critical cases, jurisdictions are considering service cuts and fare increases.

Plummeting gas prices have been good for motorists, but they cost Northern Virginia $24 million in lost revenue last year, down about 40 percent since 2013. The situation is most dire in Prince William County, where transit service is almost entirely funded by the regional gas tax, and that has threatened the viability of the bus system that serves 12,000 riders in and around the county and links the area to job centers in and around the District.

“It’s a doomed scenario,” said Prince William Supervisor Frank J. Principi (D-Woodbridge), chairman of the Potomac and Rappahannock Transportation Commission (PRTC), which covers Prince William, Stafford and Spotsylvania counties and the cities of Manassas, Manassas Park and Fredericksburg.

PRTC operates commuter and local bus service and co-sponsors Virginia Railway Express. The commission faces a $9.2 million deficit for the fiscal year beginning July 1.

“The question becomes, how do we pay for operating and capital expenses next year?” Principi said. “And the larger question is, if gas prices stay where they are now, what is the solution to the long-term sustainability of the transit system?”

In the short run, those who ride PRTC’s OmniRide and OmniLink buses should expect a 5 percent fare increase and reductions to service starting in July. But more significant cuts are possible if Prince William doesn’t approve a proposed $6 million subsidy for the system in the county’s fiscal 2017 budget, scheduled to be voted on Tuesday.

So far, other jurisdictions aren’t as pressed by the losses, in part because they have multiple revenue streams to fund transit. Still, to offset the declining fuel-tax revenue, many are tapping their general funds — putting transit in direct competition with such other priorities as education and public safety, according to government and transportation officials.

When the state’s landmark transportation bill was approved two years ago, it included built-in protections to provide stable funding tied to the state gas tax should gas prices fall below a certain level. This year, efforts to create a similar provision for the regional gas tax were approved by the Senate. But the House of Delegates failed to act on the legislation in what some local officials say was a slap at traffic-choked Northern Virginia and its efforts to boost transit.

“This is a classic man-made crisis,” said David F. Snyder, a member of the Falls Church City Council.

State legislators opposed to the provision called it a tax increase on drivers, Snyder said, but in failing to support it, they are forcing local officials to choose between using taxpayer money to offset transit costs or reduce service — and taking money from other priorities.

“We ought to be increasing transit, not reducing it,” said Snyder, who also serves on the Northern Virginia Transportation Commission, which facilitates transportation planning and funding for the region and is a PRTC partner in VRE operations. “We have highway congestion that is out of control. And we have lots of people who would take transit if it were more attractive. And then there are those who absolutely have to take transit because they have no other alternatives.”

The gas-tax revenue is used for capital and operating expenses for transit. Prince William uses it to pay for bus service. Fairfax and Arlington counties and the cities of Alexandria, Fairfax and Falls Church allocate it to Metro.

In Fairfax County last year, the regional gas-tax revenue was $7 million short of projections, officials said. The county was able to absorb the cost because that revenue is not the primary source of its transit funding, Transportation Director Tom Biesiadny said. The county also maintains a reserve fund it can tap. But that strategy isn’t sustainable if prices continue to fall, Biesiadny said.

“In the short term, we can handle it,” Biesiadny said. “If gas prices stay at $2 or below, that would be much, much more of a concern to us.”

Transportation officials are already projecting lower revenue this year, which is why Prince William is considering a 10 percent reduction in bus service, including eliminating some Friday trips, dropping service to Capitol Hill and restructuring its bus lines. Those cuts, combined with the 5 percent fare increase, would save PRTC $4 million. The annual budget for the agency’s bus division is $34 million.

PRTC officials say the slipping gas-tax revenue has only exacerbated a nearly decade-old problem it has had since losing its county funding.

Eight years ago, in the midst of the recession, Prince William eliminated its general-fund contribution for transit, leaving the county’s share of the regional gas-tax revenue as PRTC’s main funding source. That wasn’t a problem when gas prices were high. In fact, PRTC was even able to build a reserve fund during those fat years. But as prices at the pump have plummeted, that reserve has dwindled, and it is expected to be fully depleted in the coming fiscal year.

Now, the future of the transit system that serves Prince William, Manassas Park and Manassas City is uncertain. It could continue next year with the proposed service adjustments and if the Board of County Supervisors approves the $6 million subsidy. But the subsidy is tied to the approval of a budget that includes a controversial property tax increase of nearly 4 percent.

If the subsidy isn’t approved, riders will see significant service cuts, officials said.

“What we will end up doing is parking a whole lot of buses and laying off bus drivers and mechanics,” Principi said.

PRTC Executive Director Eric Marx said that the agency would be forced to end its long-haul routes to Washington and the Pentagon and feed only to Metro. That could test Metro’s rush-hour capacity on the already crowded Blue Line. It also could put more people on the congested Interstate 395 corridor.

Moving forward, officials say, PRTC needs to find a long-term solution, while the region’s leaders say they will once again push state legislators to approve a provision to protect the regional gas tax from wide price fluctuations.

Riders, not surprisingly, hope the system’s 23 routes are saved and fares kept flat. The proposed changes come on top of previous service cuts and a cumulative fare increase of 30 percent since 2009, PRTC officials say.

Art Narro, a software developer who commutes between Woodbridge and Rockville five days a week, said fares are already expensive for the bus ride to the Springfield Metro station and to get to downtown Washington, where he makes another Metro transfer.

He slugs regularly to save money, but the Omnibus-to-Metro ride has been part of his commute for more than 20 years. Raising fares and cutting bus service that is already less timely than ideal would further discourage transit use, he said. But even more disturbing, he said, is the possibility that service would disappear altogether.

“The buses are good. The drivers are very friendly. They do a good job,” he said. “At the very least, keep the current service.”