“Out here, it’s cars,” Mr. Dion said. “Cars, cars, cars, cars.” Doctor appointments, grocery shopping, movie theaters, even reaching the beach from where the Dions live all require a car. “If you don’t have a car, you’re stranded,” said Mr. Dion, a retired marriage and family therapist. He and his wife, a master weaver, moved to California 38 years ago from Connecticut.

While he still drives 16 miles — on three freeways — to writers group meetings, he has decided the time has come to plan for a carless future.

On a recent Thursday afternoon he took a first step in that direction. He and his wife, Rosemarie, also 81, drove to the Grossmont trolley station, part of the Metropolitan Transit System, and rode the green line toward Old Town, where they spent part of the day exploring. They traveled in a group led by Judi Bonilla, a gerontologist and founder of We Get Around. The organization is a fledgling nonprofit that promotes the use of public transportation for adults who believe they may be near the end of their driving days.

But in places like San Diego, the transition is not an easy one. From their home, the nearest bus stop is a mile walk. “I can still do it,” Mr. Dion said. But his wife cannot.

“All of this is well and good while you have your health,” he added. Yet, he allowed, “You can’t do all the things you used to do.” The Dions contemplate relocating but so far haven’t taken any action.

“She likes San Diego,” Mr. Dion said of his wife. “I don’t know anyone in Connecticut anymore.” Their grown children live in San Francisco and Illinois.

The situation the Dions face now is likely to become more common as aging baby boomers age even more. During retirement planning, transportation is often an afterthought. Yet, figuring transportation into plans is essential, experts say.

According to the American Journal of Public Health, Americans are outliving their ability to drive safely — a woman, on average, by 10 years, a man by seven. Over all, the ability to drive safely as one ages depends on health. Some people can drive into their 90s while others begin to cut back at 65.

And yet, most people prefer not to think about the day when they have to rely on others or use public transportation for routine activities. “People avoid the topic,” said Beth Shapiro, a licensed clinical social worker in Rockville, Md., who runs the Jewish Social Service Agency’s “To Drive or Not to Drive” program.

“When people make retirement plans, they make no transportation plans because they assume they’re going to drive forever,” said Katherine Freund, founder and president of the Independent Transportation Network, a nonprofit organization that provides rides for older adults, with 27 affiliates throughout the country. Nationally, for those over 65, 2 to 3 percent of what distance they travel is on public transportation, 8 percent on foot and the rest by car, Ms. Freund said:

Not driving by choice is different from realizing you are no longer fit to drive. Deciding to drive less typically happens incrementally. People might decide to stop driving at night to unfamiliar places, for instance. But regardless of the reason, not driving can limit your autonomy, even your social life, depending on where you live.

Even in places like Washington, D.C., which has a strong public transportation network that extends into suburban neighborhoods, it can be “that last mile” that is the hardest, Ms. Shapiro said. “You can get most of the way there.”

But getting from public transportation to your final destination or walking a mile or more to a bus stop could present an insurmountable challenge, especially on freezing winter days or hot, muggy stretches, she said. For some, getting on and off buses could be an obstacle.

When planning ahead, think about whether you prefer to stay in your community, plan to downsize or will relocate. According to a 2014 AARP study, by age 65 and older, 87 percent of people want to remain in their current community as they age. Financial and family considerations play a role in decisions about where to live.

“If you’re 55, you have to project out into the future,” Ms. Bonilla said.

In car-oriented areas like San Diego, people often rely on a network of family and friends for transportation. But there aren’t always younger family members available to drive those in their 80s and 90s. Sometimes, family members live in another city or state.

Transportation is the second highest household expense after housing, according the Office of Planning, Environment and Realty, which is part of the Department of Transportation’s Federal Highway Administration.

Those living in households that are car-dependent spend 25 percent of income on transportation. By living closer to work, shopping, restaurants and other amenities, households can reduce transportation costs to 9 percent of their total income.

The Independent Transportation Network requires riders to fund a personal transportation account in advance; riders receive a monthly statement detailing all payments — charges that are often lower than using taxis. Drivers assist riders in reaching their destinations and with packages. No money is exchanged during the ride and tips are not permitted.

Potentially filling the void are a number of new transportation services that provide rides for a fee, including Uber, Lyft and Sidecar. Some senior housing communities have shuttle buses that take residents to medical appointments; each one is different, so it is important to check when you are considering places to live.

Whatever decision you make about where to live and transportation, here are some guidelines from experts:

ANALYZE your current neighborhood in terms of where you typically need and want to go, and determine how you might reach those places if you weren’t driving. Include leisure activities like classes, entertainment and simply meeting friends. “Think about how you’re going to do that when you can no longer drive,” Ms. Bonilla said. “Lay out a grid and see how far these trips are from your home. That will determine where you live, whether you stay in your home.”

LOOK AT the social support where you live. “Think about your network of friends, family, fraternal and faith-based organizations because those are the places where you have established relationships,” said Ms. Bonilla, who is 57.

CONSIDER becoming a volunteer driver through an Independent Transportation Network affiliate as Jacqueline Masumian, 67, a retired landscape designer from Westport, Conn., has. “I chose to make a plan if I become incapacitated or my eyes fail me,” said Ms. Masumian, who lives with her husband. By driving others, she builds credits for rides if the day comes when she is no longer able to drive. “Here it would be impossible to live without a car,” she said. “If I’m old and alone I thought I’d like to have somebody drive me around.”

Joel Beckoff, who turns 59 this month, also volunteers as a driver once a week through the Coastal Connecticut affiliate of the Independent Transportation Network. A certified public accountant who worked for various corporations during his career, Mr. Beckoff retired a year ago. He doesn’t expect to use the stored driving credit, but “it’s nice to know it’s there,” he said. He and his wife, Arline, have considered moving to Manhattan someday, where public transportation seems limitless, compared with most places in the country. “When I can no longer drive, I don’t expect to be living in suburbia,” he said.