WASHINGTON - The ride in Betty Lee Thatcher's snazzy red Volkswagen Beetle was short and uneventful. But it meant everything to Patricia LaRue.

LaRue, who never learned to drive, traveled everywhere by bus, train or subway, including to her job at the State Department. But since her cancer was diagnosed last year, LaRue has needed a little extra help getting around Northern Virginia.

LaRue, 60, has to rely on friends or volunteers for door-to-door transportation, so she signed up for a ride with the Annandale Christian Community for Action, which coordinates volunteers from 27 Fairfax County, Va., churches to ferry senior citizens to medical appointments.

But the organization is running out of drivers because so many volunteers are too old to drive. Thatcher, who drove LaRue to a doctor's office Tuesday, is 80. LaRue's driver for a follow-up visit is 87.

"I'd say the average age is 89 and rising," said Nancy Hall, president of the group. "We just don't have the younger folks because they work longer. Or maybe they're not as charitably minded."

With the Jack Kerouac generation well on the road toward retirement, demographers and experts on aging are worried that retirees will become increasingly stranded in the suburbs as they stop driving and are urging policymakers to invest in new public transportation options.

The number of senior citizens is expected to double by 2030. As that population swells, experts said, so will the need for new ways to get around as more people live well beyond the age when they quit driving. A 2002 study by the National Institute on Aging found that about 600,000 people 70 or older stop driving every year and become dependent on other forms of transportation.

A substantial number of older Americans already have difficulty getting where they need to go because they no longer drive. This is true even in areas with a host of transportation options, experts said.

"We have data that show people are stranded," said Elinor Ginzler, AARP director for livable communities.

More than 20 percent of Americans age 65 or older do not drive. Of those, more than half - about 3.6 million people - stay home on any given day because they have no transportation, AARP says.

Those who cannot get around become isolated, and isolation can have serious consequences on a person's mental and physical well-being. For example, AARP says those who are unable to find transportation make 15 percent fewer trips to the doctor.

Many older people do not use mass transit, often because of real and perceived obstacles, Ginzler said. The multitude of services can frustrate senior citizens - and others - who struggle with bus schedules and multiple transfers. Such obstacles help explain why nearly 90 percent of older residents use private vehicles.

Governments and nonprofit organizations across the nation have explored strategies to help seniors get around, including implementing new approaches to volunteer driving services, revamping bus lines to make them more flexible and redesigning streets and highways to accommodate older motorists and pedestrians.

In Maine, for example, Katherine Freund started a nonprofit organization to help seniors get around. Through Independent Transportation Network and ITNAmerica, which is working to replicate her model across the country, younger volunteer drivers can get credits to receive rides later or donate them to others. Residents who stop driving can donate their cars for credits, and merchants can help pay for rides for older customers.