Ridership is surging on the Twin Cities transit service for people with disabilities, and the cost of those Metro Mobility trips is consuming an ever-bigger share of state funding for metro-area transit.

The Metropolitan Council expects the number of rides on the federally mandated service will climb to 2.9 million by 2020, about double what it was in 2010. Absent a fix, the projected cost of those trips — $89 million — could force the agency to reduce regular bus service.

Metro Mobility offers customized, shared-ride transit service to 40,000 people certified as unable to use traditional buses and light rail. A graying and growing population is likely fueling demand, and the Met Council, which oversees Metro Mobility, is one of many transit agencies across the country grappling with rising costs for highly subsidized “paratransit” services.

“They talk about the ‘silver tsunami’ as what’s coming, and I think we’re definitely on the front edge of it,” said Michael Richter, owner of Transit Team, one of Metro Mobility’s contracted providers. “And I think it’s going to continue to grow.”