When should you take the car keys from an aging parent?

“We have occasionally had people threaten to hurt family members who said they were not safe to drive,” said Lissa Kapust, the director. “You get the range of emotions, from people who say they refuse to stop driving, to those who are unhappy but swallow the bitter pill.”

Last week, on June 16, the Gerontology Institute at the University of Massachusetts Boston convened an older driver safety summit, a “call to action” as the state’s over-65 population is growing at a record pace, and car accidents involving older drivers account for one in five crash-related hospital stays in the state, and 12 percent of crash fatalities.

But with 70 percent of Americans over 50 living in suburban or rural areas with little or no public transportation, a person’s home is not just his castle “but his prison,” said Joseph Coughlin, director of MIT’s AgeLab.

Elizabeth Dugan, a UMass Boston gerontologist and a meeting organizer, hopes it will spur action aimed at reducing fatal accidents for people over 65. Older drivers have a lower crash rate than drivers 25 and younger, she said, but are more likely to die or be seriously injured in a crash.

Don Blackburn, 85, of Framingham, a speaker at the summit, stopped driving a few years ago — but only after family urging. “I knew right away I wasn’t going to win,” he said offstage, recounting family arguments.

“There is a sense of loss: of your identity, you know, and independence,” said Blackburn. “Then again, you don’t want to be responsible for hurting someone.”

It’s that very fear that propels many adult children. A Winthrop woman, 40, a writer and tutor who asked for anonymity, said she and her younger sister are so worried their father will drive they regularly check his car’s odometer.

“We’re in the middle of it,” she said, briefly casting her mind back to the beginning, before her father was diagnosed with Alzheimer’s, before he lunged at her when she temporarily took his car keys.

She recalled her childhood in southern New Hampshire, when a drive with dad was a happy subject. “He would take us on ‘mystery rides,’ ” she said. “He’d pile us into the car and not tell us where we were going, and we’d end up at swimming hole or a park.”

The sisters have studied all the advice, she said, “But there’s nothing that prepares you for a perfectly mild-mannered man to lunge at you.”

The difficulty of taking the car keys from a parent is captured in a pamphlet put out by The Hartford insurance company.

“Some older drivers will not respond to constructive conversation,” the pamphlet advises. “You may have to consider disabling the car, filing down the keys, or taking away the car. Some older drivers, however, find ways to work around these actions, such as calling a mechanic and having a disabled car repaired.”

In 2010, as a provision of the Safe Driving Law, the state tightened rules for drivers 75 and older, and began requiring them to renew in person — and not online — and pass a vision test or present a vision-screening certificate.

There is no legal requirement that health care providers report to the RMV drivers whom they believe are not physically or mentally capable (although the Massachusetts Commission for the Blind is required to maintain a registry of the legally blind).

But advocates say the law did two important things: It stated that medical professionals who report drivers cannot be held liable for doing so, and it also compelled the RMV to develop regulations designating cognitive or functional impairments likely to affect the ability to operate a vehicle.

Coughlin, the AgeLab director, said that about 26 or 27 states have age-related testing requirements, but the issue is complicated. “Scientifically we don’t have a perfect test,” he said. “We don’t even know how old is ‘older.’ In some states, it’s 50, in others, it begins at 85. What most people consider old is 20 years older than they are.”

Experts advise family members to have a transportation plan in place before starting the conversation — and scapegoat Boston’s notoriously bad drivers. But a Boston man who battled his father for years said his dad didn’t want to hear about it. “He refused to use the shuttle service for senior citizens,” he said.

The man, an employee at a local university, asked to remain anonymous, but his family’s experience will feel familiar to many: There was a fender bender, and mounting near misses. The family became increasingly afraid that the father would hurt himself or others. After he lost his license — following a hospitalization — they prayed he would fail the follow-up road test.

He did fail, and afterward sat distraught in a McDonald’s with another son. “Dad’s crying like a baby,” the man’s brother texted him. “He’s saying his life is over.”

As hard as it is to lay down the rules, it may be harder to hear them. A retired health care worker said that when her family told her to stop driving — she’d had an accident — she thought: “You’re not taking my keys away.”

But after a second accident, she feared they were right. “I started to think I could be one of those people who goes through a post office and makes headlines,” she said. “You feel so sorry for those people.”