Figuring out how to stop older drivers with dementia from driving before they hurt someone has proven challenging for state leaders, and a new study aims to help guide those efforts.

Research from the University of Pittsburgh's Graduate School of Public Health casts doubt on one approach that makes intuitive sense — requiring doctors to report patients with dementia to licensing authorities — but provides some evidence that other rules can help seniors in certain age groups avoid accident-related injuries.

>> Read more: The best ways to get older drivers to give up the keys.

The study, published Wednesday in the journal Neurology, analyzed data from 136,987 drivers across the nation aged 60 and up who were hospitalized after a motor vehicle accident.  Of those, 5,564 were diagnosed with dementia.  The study looked at whether states with certain driving rules had a lower proportion of drivers with dementia who had been hurt badly enough in accidents to require hospitalization. The data covered 2004 to 2009.

In those years, three states — Pennsylvania, Oregon, and California — required doctors to report patients with dementia.  In addition, 27 provided legal protection for doctors who reported patients.  In New Jersey, doctors are required to report certain health problems but not necessarily dementia.

Contrary to what the researchers expected, required reporting had no impact on hospitalization rates for drivers with dementia. Two things made a difference for drivers aged 60 to 69: requiring drivers to renew licenses in person, and to take vision tests. Hospitalized drivers in states with in-person renewal were 37 percent to 38 percent less likely to have dementia.  They were an additional 23 percent to 28 percent less likely to have dementia in states that required vision testing at in-person renewal.  Road testing also helped, but only for drivers age 80 and older.

About 10 percent of people over 65 have dementia, but it is much more common among people over 80, so reducing rates among people under 70 likely affects only a small part of the problem.

New Jersey drivers renew their licenses every four years.  They can renew by mail every other time. Vision tests are not routinely required. Pennsylvania does not require in-person renewal, but about 1,900 drivers a month over 45 are randomly told to take an eye exam.

The study is only one piece of the puzzle.  It examined one type of available data in a realm where data are fragmented and difficult to obtain, said Yll Agimi, who worked on the study while he was a doctoral student at Pitt and is now a health data scientist at Salient CRGT Inc.

Some researchers may care more about the total number of accidents, injuries to other drivers or pedestrians, or deaths. Other studies have found that in-person renewals and vision tests reduced traffic fatalities among the oldest drivers — those 85 and up. One study found that insurance claims declined after Illinois required drivers over 75 to take a driving test when they renew their licenses.

The researchers were most surprised by the results on physician reporting. It's possible that physicians aren't actually following the law or that licensing authorities aren't following through often enough on reports.

"The enforcement piece is a big issue," Agimi said.

Steven M. Albert, chair of the department of behavioral and community health sciences at Pitt, said that taking the keys from an aging driver is one of the most difficult issues that doctors and families face.  The study shows how difficult it is to affect those decisions.

It also shows, he said, that "we're not getting the extra bang out of the buck in requiring physicians to report patients with dementia to the department of motor vehicles."

Keeping unsafe drivers off the road likely will require greater cooperation between families and health care providers.

"I don't think we can expect physicians alone to be the gatekeepers to safe driving and licensing. It's probably an unrealistic demand," he said. "It's a general public-health education issue that people need to know, that cognitive disorders affect driving."