Motorists with diabetes or epilepsy run a one-third higher risk of accidents, but tougher restrictions on their driving are probably unnecessary, a study today concludes.

Both of these common diseases can cause people to black out while behind the wheel, and doctors assume this is why such motorists are more prone to auto crashes.All states require people with epilepsy to stay off the road for a few months or even years after their last seizure. In most states, diabetic drivers also need letters from their doctors assuring that they are not prone to blackouts.

"There is a slight increase in risk for both epileptics and diabetics, but the increase is small enough that we didn't feel there was any great need to change current driving restrictions," said Dr. Phiroze Hansotia of the Marshfield Clinic, who directed the study.

Dr. Richard Kahn of the American Diabetes Association said diabetics should be evaluated individually to see whether they can safely drive.

"There are people with diabetes who should not be drivers," he said. "But the overwhelming majority could be drivers and be as safe as people without diabetes."

An estimated 14 million Americans have diabetes, and 2.5 million have epilepsy.

The study was based on a review of all the car accidents and driving violations among 30,420 people who lived within 25 miles of the Marshfield Clinic in rural Wisconsin. Among these people were 484 with diabetes and 241 with epilepsy.

Virtually everyone in the Marshfield area is treated at the clinic, so the researchers believed they could identify nearly all residents with those diseases.

They found that epileptics had a 33 percent higher accident risk and diabetics a 32 percent higher risk than the rest of the population. Both groups were also more likely to be stopped by the police for careless driving.