Most Maryland motorists are making illegal right turns at red lights -- by not coming to a full stop before turning -- according to the findings of a two-year study of 13 intersections in Montgomery and Prince George's counties.

Although the study found that few of the illegal turns were actually dangerous, the report said they nonetheless pose " a serious potential threat to life and property."

The findings also appear to support those of a recent national study, which concluded that right-on-red turns have caused a 20 percent increase in the number of accidents and a 57 percent increase in the number of pedestrians struck by cars. The figures from the national survey, done by the Insurance Institute for Highway Safety, have been questioned by federal and Maryland transportation officals. But few if any other major studies of right-on-red turns have been done since the majority of states began permitting them in the mid-1970s.

Neither Montgomery nor Prince George's has any statistics to show whether right-on-red turns are causing more accidents with other vehicles or more injuries to pedestrians. Many, if not most accidents in the two counties go unreported because state law does not require motorists to report minor accidents if both cars can be driven away under their own power.

The two-county study found that 64 percent of motorists approaching 13 intersections did not stop, as required by state law, before making a right at red light. This illegal turn rate is so much greater than that reported in other sates "that it is cause for some concern," according to the author of the report, William E. Baumgaertner Jr., a former Maryland state and Anne Arundel County traffic engineer who now works with a private firm in Atlanta.

Baumgaertner's study, done for the Journal of the Institute of Transportation Engineers, found that the rate of violation appears to be increasing, but it also found that only 2 percent of the 5,000 right-on-red turns studied were actually unsafe and might have caused accidents.

Baumgaertner concluded, however, that when 64 percent of motorists don't come to a full stop at a stop light, that "indicates a serious potential" danger.

The study, done at intervals between late 1976 and July 1978, focused on six intersections in the College Park-New Carrollton section of Prince George's County and at seven Montgomery intersections.

Right turns on red are permitted at about 70 percent of Montgomery's 400 signalized intersections, and at Prince George's 475 intersections.