GEICO CORP., one of the country's major insurers, has stopped writing automobile policies for people who have radar detectors in their cars. Its reasoning is simple enough: speed is related to auto accidents, and people who use radar detectors are giving about as strong an indication as one could want that they're likely to speed; there's no other reason to shell out $300 or so for the things.

This week various defenders of radar detectors -- manufacturers, a citizens' group, an auto magazine -- appeared at a hearing before Maryland's insurance commissioner asking that Geico be forced to abandon the restriction in that state. They pictured users of radar detectors as an eagle-eyed, alert lot, mostly motoring enthusiasts (in the words of one lobbyist, the sort of people who are more "aware of their surroundings") proceeding briskly and competently past that lesser breed that lets itself be cowed by a wimpy 55 mph speed limit. There was little room in their picture for those who blitz residential streets at 60 mph (an egregious sort of offender against whom police radar can be particularly effective) or who tailgate at high speed and weave in and out of Beltway traffic. Geico produced studies showing what common sense and a little experience on the crowded highways of this region make clear to most of the rest of us: that drivers with records of speeding are more likely to be involved in accidents.

Radar detectors are illegal in the District of Columbia, Virginia and Connecticut, but attempts to ban them elsewhere have foundered. They are a big business now, sold in discount department stores and advertised in national publications. In 1986 a proposal in Congress to ban them was quickly buried in a sea of telegrams, phone calls and letters.

But the fact remains that their sole purpose is for use in breaking the law, a law directly related to public safety, and if insurers consider the people who use them a bad bet, they have good reason for doing so.