Motorcyclists wearing helmets receive 23 percent fewer head injuries than unhelmeted cyclists, according to a new study conducted for the Department of Transportation.

The study, performed by a University of Southern California safety expert, also said that the major cause of motorcycle accidents is the failure of motorists to see motorcycles in traffic.

The detailed analysis of 900 accidents in the Los Angeles area over the past two and a half years by Prof. Harry Hurt was designed to determine the causes of motorcycle accidents and injuries and methods of reducing both.

Some 45 percent of all motorcycle accidents involved an automobile or other vehicle turning left into the path of a motorcycle, the study showed.

Of the 900 accident reports in the sample, there were 54 fatalities. Eighty-five percent of those victims were not wearing helmets.

Last June, the National Highway Traffic Safety Administration released data showing that 4,082 motor cyclists were killed in traffic accidents in 1977, an increase of 23 percent over a year ago.

NHISA administrator John Claybrook said the results of the USC study represented new evidence that helmets, and helmet laws, can help save lives.

"The value of helmets has been documented for more than 30 years," Claybrook said. "In 15 states that repealed their helmet use laws since 1976 and that report whether cyclysts involved in accidents were wearing helmets, the number of fatalities involving helmetless cyclists showed a dramatic increase of 88 percent from 1976 to 1977."

Claybrook has been deluged at recent NHTSA town meetings across the country with requests from motorcycle clubs to prod NHTSA into moving faster on the issue of motorcycle safety regulation.

Yesterday she urged motorcyclists to wear yellow jackets or clothing to make them more visible in traffic.

Other findings of the USC study show that:

More than half of the drivers surveyed had less than six months experience with the bike involved in their accident, although total bike-riding experience averaged more than three years.

About 92 percent of the respondants had no formal or professional training in motorcycle riding.

Nearly 12 percent of all accidents involved some alcohol consumption by the driver, but about 53 percent of the fatal accidents involved a motorcycle driver who was drinking.

Forty-one percent of the accidents are caused by judgement errors on the part of motorcyclists, or in "man machine relationships." Vehicle failures account for only about 3 percent of all accidents.

About 12 percent of the cyclists surveyed either had no license or were riding with a revoked license.

Females constitute 2 percent of the cycle rider population, but are involved in 3.8 percent of the accidents.