WHEN PEOPLE decide which cars to buy, sometimes they wonder what would happen in an accident. But in the past, reliable safety data have been very hard to find. That's why Congress, some years ago, told the Transportation Department to develop a uniform system of safety indicators for each model of every car, similar to the gasoline mileage figures the government publishes. The test crash results for small cars, announced this week, are a preliminary step in producing that safety index to help the buyer.

Beginning late next year, any car sold in this country must pass a basic crash test. Hitting a fixed wall at 30mph, the car must provide standard protection to a dummy with its seat belt fastened. Now the Transportation Department is conducting crashes at higher speeds to see which models offer more than minimum protection.

On the fuel efficiency tests, it was generally the Japanese and German cars that were the winners. On the crash tests, American cars have been superior. Among similarly efficient small cars, there turn out to be important differences in safety.

Coming at this moment, these tests inevitably carry a certain political overtone. The Carter administration is struggling wildly to find ways to help the American automobile industry stand off the imports. But safety testing, and public announcement of the results, are a perfectly legitimate froce in the automobile market. Particularly for that industry, it's highly desirable to generate competitive pressure for safety performance. That can only lead to better cars, while import restrictions will produce worse ones.

The tests also show that, as everyone knows, big and heavy cars tend to be safer than small and light ones. Does it mean that people are wrong to shift to small cars? One answer lies in the kinkd of improved engineering toward which these tests are already pushing the manufacturers. Another answer is that the seat belt is a great equalizer. The Transportation Department recently set up a head-on collision between a big car and a little one, each going 30mph. The dummies in the little car had their seat belts on; those in the big car did not. If they had been real people, the occupants of the little car would probably have walked away from the crash. Those in the big car would, unfortunately, have been carried out.

A final note on safety and small cars: with more of them on the road, and much wider diversity between large and small vehicles, the case for the nationwide 55mph speed limit is stronger than ever. It's essential protection to a public that can no longer rely on sheer size and weight to protect itself on the highway. The Republican Party, which endorsed repeal of the national speed limit last month, deserves a ticket for reckless driving.