Traffic deaths nationwide over the Labor Day weekend dropped about 10 percent from 1986, marking the third consecutive holiday weekend in which highway deaths have dropped this year despite the increase in the speed limit to 65 mph on many segments of the interstate highway system.

The National Safety Council, a nonprofit research organization in Chicago, said 441 motorists died between 6 p.m. Friday and midnight Monday. Over the same weekend in 1986, 487 traffic deaths were reported. The organization reported similar declines over the Memorial Day and July 4th weekends this year.

The seemingly contradictory trend of higher speeds and fewer fatal accidents is also reflected in separate data compiled by the National Highway Traffic Safety Administration. Preliminary figures show that highway deaths nationwide during June and July dropped about 7 percent from 1986.

These reported declines in traffic fatalities came after 37 states took advantage of congressional authorization to raise the speed limit from 55 mph to 65 mph on predominantly rural sections of interstate highways.

But traffic safety experts said yesterday that it is too early to say whether the lower death figures are a genuine piece of good news or a short-term statistical aberration.

"Any kind of {year-to-year} comparison in a 60- or 90-day period is statistically invalid," said John Cook, senior vice president of the Insurance Institute for Highway Safety, an insurance industry research unit. Cook said the public should be skeptical of conclusions drawn from the highway death rate in the months since states began raising speed limits.

Although many public and private organizations study highway fatalities, there is a good deal of befuddlement about this year's trend. For that matter, experts do not even agree on the definition of a highway death.

The federal government records a traffic death in any case where death from an injury suffered in an auto accident occurs within 30 days. The National Safety Council and some other private groups use a 90-day standard.

Moreover, there is no clear information on accident rates on rural interstate highways, the only place the 65 mph limit is permitted. In Illinois, state police say traffic deaths on rural interstate segments have declined from 1986 levels in the three months since the speed limit was raised; in New Mexico, the number of fatalities has doubled. Students of traffic safety have offered several explanations for the situation.

The Federal Traffic Safety Administration cites data showing that the use of seat belts is up sharply this year, possibly because of new state and local laws requiring their use. Crackdowns on drunk driving also may affect the fatality rate, experts suggest.

The federal agency and the National Safety Council are studying the impact of the speed limit increase on traffic safety. Preliminary results should be available next March, federal officials say.

Congress passed legislation April 2 permitting the increase in speed limits on segments constituting about 73 percent of the interstate highway system.

Since then, according to the National Conference of State Legislatures, every state west of the Mississippi (except Alaska, which has no roads that qualify) has raised rural speed limits, as have Alabama, Arkansas, Florida, Illinois, Indiana, Kentucky, Louisiana, Maine, North Carolina, New Hampshire, Ohio, South Carolina, Tennessee, Vermont, West Virginia and Wisconsin.