After years of hand-wringing over the source of its plummeting ridership — teleworking, a shrinking federal workforce, lower gas prices, ride hailing, bike sharing — a consultant concluded what has been obvious to the hordes of riders who have abandoned the system.

It’s the service.

Metro turned to a consultant late last year to delve into the issues driving its faltering ridership, which by May 2018 had tumbled well below 2009 levels of 750,000 average weekday trips to just over 626,000.

Transit ridership is trending downward in major cities across the country, but Metro’s ridership slide was particularly dramatic — with trips falling nearly 10 percent between May 2016 and May 2018. Now Metro says two-thirds of the losses over the last two years have come during off-peak hours, when rebuilding efforts make riding difficult and the accompanying disruptions drive customers away.

The model, developed by consultant VHB, found the factors that “best correlated” to ridership changes are service-related: train frequency, the number of trains in service and the proportion of trips delayed beyond periods of 10 minutes or 30 minutes, according to Metro.

“Not surprisingly, when the number of trains serving a station increases, ridership increases; and when the percentage of trips delayed increases, ridership decreases,” Metro spokeswoman Sherri Ly said.

“Public transportation only works if three things come together: Convenience, reliability and affordability,” Metro board chairman Jack Evans said. “That’s it. You don’t need consultants to tell you this.”

The study cost $460,000.

Metro says the modeling tool can help in crafting its upcoming fiscal year 2020 budget, when fare hikes will be back on the table. The last round of increases and accompanying service cuts in 2017 incensed riders already fed up with chronic service disruptions.

Insights from the consultant’s analysis were gleaned from discussions with individuals familiar with the work and were confirmed by Metro.

The VHB analysis homed in on several factors Metro had previously identified as driving ridership. For example, it “confirmed that ridership is a function of service quality/quantity, development around rail stations and along bus corridors, track work, events/weather, and gas prices,” Ly said. “The finding that service quality and reliability is critical to ridership confirms the importance of many Back2Good initiatives that resulted in 80 percent customer satisfaction last quarter for the first time since 2014, as well as transit-oriented development near and around stations.”

(Metro introduced a program called the “Rush Hour Promise” this year that refunds fares to riders’ SmarTrip cards if their trips are delayed more than 15 minutes from Metro projections.)

The finding may seem obvious but could prove deeply consequential for an agency already facing operating constraints. Metro must adhere to a new 3 percent cap in growth in its operating subsidy under Virginia’s new dedicated-funding law.

The agency’s financial situation is complicated by an arbitration ruling last month ordering Metro to provide pay raises to its workers. Metro’s arbitrator, citing its revenue challenges, concluded the agency does not have the money.

The majority opinion of the arbitration panel outlined Metro’s fear of a “death spiral,” in which a lack of capital investment would lead to ongoing service problems, further driving away riders and deepening Metro’s revenue hole.

Metro says its average weekday ridership is stabilizing. In July, average weekday trips totaled 644,000, a 2 percent increase from the previous year — even when a partial closure hit the Red Line late that month.

The 2020 budget proposal, Metro said, will see the model’s debut. It could be used to answer hypothetical questions around a potential fare hike, for example.

Evans, a proponent of a $2 flat fare, said he wants the region to consider a plan that would lower fares, not raise them.

He acknowledged however, “there is no silver bullet. . . . There is no short term solution that gets people back.”

The problem is, Metro needs to make a turnaround. The agency is under pressure from state and local governments, arbitrators and credit rating agencies to increase ridership. Moody’s, which downgraded Metro’s bond rating to A2 in 2016, affirmed that rating in August with one notable change. On the heels of securing dedicated funding commitments from around the region, Metro’s outlook was changed from “stable” to “positive.”

But Moody’s noted that ridership remains a key challenge for Metro.

In one scenario, Metro’s bond rating could be upgraded in the event of “stabilization or improvement in ridership that increases predictability of passenger fare revenue and enhances WMATA’s operating flexibility.”

In another, Metro’s rating could fall further in the face of a “continued downward trend in ridership.”

Metro’s revenue picture also is complicated by Uber and Lyft, which have provided little information about how many passengers they are transporting in the region on a daily basis.

While the VHB tool has the ability to account for ride hailing’s impact on ridership, the lack of available data has prevented Metro from conducting such an analysis. New data-sharing requirements set to take effect in October could solve that problem, however.

Nat Bottigheimer, a former assistant general manager for planning and joint development at Metro, illustrated the agency’s challenge this way:

“There’s a very clear relationship between stabbing and mortality. The theory is pretty straightforward — [a certain percentage] of [stabbings] result in death. For here, there’s really not science on that, and there’s also not enough” data from ride-hailing companies.

Metro last estimated that at least 30 percent of its rail ridership losses were caused by service and reliability problems, with the balance attributed to factors such as telework, federal workforce reductions, ride hailing and the rise of other alternatives.

Bottigheimer said the answer to Metro’s ridership problem boils down to a simple strategy.

“If you focus on service and provide good reliable service, then transit is making the argument for itself,” he said. “If you’re delivering service that is valuable for the money then people want more of it.”