THE WELCOME but perplexing news is that fatalities from auto accidents declined 10 percent over the last year, and the rate of deaths from auto accidents per miles driven is the lowest since records began to be kept in 1925. There were 46,300 traffic deaths last year--5,000 fewer than in 1981-- although Americans drove 1 percent more miles. In 1981 there were 3.2 traffic deaths per 100 million miles driven and in 1982, 2.95.

No one is quite sure why. Some give credit to the recession. But that only explains why there was little increase in miles driven. Others point to safety improvements in cars. But energy-absorbing steering columns, seat belts and laminated windshields were required long ago and showed up in a sharp drop in death rates in the early 1970s. Today, Americans are slightly more likely than they were a few years ago to drive smaller cars, which statistically are less safe. Yet the death rate declines.

Two explanations seem more helpful. One is that the age of the average driver is rising as the baby boom crop grows older and the number of teen- agers drops. Young people--especially young single men--are involved in a vastly disproportionate number of accidents and traffic deaths. Also, the nationwide crackdown on drunk driving almost surely has made some difference. Hundreds, perhaps thousands, of Americans are alive today who would be dead but for the campaigns against drunk driving by law enforcement agencies. These efforts have proved themselves as much as any public policy can, and they should be continued.

But these explanations still don't account for the magnitude of the change. The traffic death rate per million miles driven has dropped sharply over the long term: 2.95 in 1982, 4.3 in 1972, 5.3 in 1962, 7.4 in 1952, 10.6 in 1942, and 16.1 in 1932. If the traffic death rate were as high last year as it was 50 years ago, 252,000 Americans would have died on the road. Roads and automobiles are obviously a lot safer today, but obviously drivers have gotten more used to automobiles and more skilled at handling them.

Our further hunch is that the national attitude to driving cars has become less romantic and more utilitarian. Drivers who just want to get from place to place drive more safely than those who are acting out some fantasy with the aid of tail fins or tachometer. That would help explain as well the reluctance of so many Americans to buy new cars--a reluctance that baffles the market researchers of Detroit and Germany and even Japan as much as the sharp drop in traffic deaths baffles the experts on that unavoidably engrossing subject.