IF YOU intend to drive in the District of Columbia in the near future you'd better start untangling the seat belt that's jammed between the cushions and twined around the gizmo that moves the front seat forward. The D.C. Council has just passed mandatory seat belt legislation, and as soon as the mayor signs it and Congress has 30 days to review it, the new law will go into effect. Anyone riding in the front seat of an automobile without wearing a belt will be subject to a fine. The law is a good one. Experience in other states has shown that requiring this simple safety measure reduces the rate of death and serious injury on the road. New York was the first state to require seat belt use, and in three months traffic fatalities dropped 27 percent.

Washington's new law should be viewed in a national context that is interesting and a little bit complicated. The Department of Transportation has issued regulations that will require the manufacturers of new cars to install passive restraints -- either air bags or automatic seat belts, the kind, for example, that connect when the door is closed. But the manufacturers have an escape clause: the requirement will not go into effect if, by April 1, 1989, states that include two-thirds of the nation's population have passed mandatory seat belt laws. This explains why the auto industry is now busily working to get these laws passed all over the country. Fifteen states now have such statutes. In California, though, highway safety supporters didn't want the state seat belt law to be used by DOT as a reason for abandoning air bags and passive restraints. That state therefore specifically provided that its seat belt law would be automatically repealed if it were used by DOT in determining whether the requisite number of seat belt statutes had been passed. The D.C. council put the same provision in the local law and consciously set the fine at $15, too low to meet the federal standard.

Even if the air bag regulations go into effect 31/2 years from now, it's a good idea to have seat belt requirements too, for they apply to all vehicles, not just new cars. It's hard to understand why people don't voluntarily use seat belts, but they don't. Safety experts estimate that half the people killed in highway accidents would have survived if they had been wearing the protective devices. There are always some people who resist a directive to do something for their own good. But the new law should provide a powerful incentive for the careless and the preoccupied to make a habit of taking this simple safety precaution.