There was a time when the major traffic menace was the incompetent driver: the motorist who had trouble keeping in his lane, who didn't know how to use a rear-view mirror, who turned without signaling (or signaled without turning) or who became hopelessly befuddled at intersections with both full-green and left-turn arrows.

Now there is a more dangerous breed: the driver who knows that what he is doing is illegal but who does it anyway. He runs red lights, turns whenever he gets tired of waiting for the light to change, treats stop signs (if he observes them at all) as caution lights.

The breed is increasing, and no one seems to know why. Some blame it on the institution here of right-turn-on- red -- the idea being that once people get in the habit of proceeding on a red light, they lose respect for red lights. But I first remember reading about traffic "rule trashing" in The New York Times, and New York City does not permit right on red.

Some blame it on lax police enforcement. If the word gets around that police aren't ticketing drivers who run red lights or make illegal turns, the number of violations will increase. But local authorities say they are not only handing out more tickets but also have raised the cost of the tickets. Still, for reasons unexplained, the lawlessness increases.

But if the causes are obscure, the effects are plain -- and growing. The guy running the light could kill some innocent pedestrian, biker or fellow motorist, falsely secure in the knowledge that he had the right of way. The secondary effect may be even more frightening. That's the obvious danger. Less obvious, but perhaps more unsettling, is this: law-breaking produces law-breakers. The more you see people break the rules without penalty, the more tempted you are to break a rule.

And it gets worse. "When the number of rule-breakers reaches a critical mass," Susan Jacoby said in a New York Times piece a year and a half ago, "social pressure -- the prime enforcer of civilized behavior -- works on behalf of, instead of against, lawlessness."

Hers is a profound observation, and one that goes far beyond driving. Because of our tendency to equate permissible behavior with what people are actually doing, we keep expanding the limits of permissibility, and finally develop contempt for those who insist on obeying the old rules.

In the case of driving, it used to be the scofflaw -- the light-taker or speeder -- who earned our opprobrium. But as the number of scofflaws increased, so did the pressure on erstwhile law-abiders to cheat in order not to be routinely disadvantaged.

When the number of cheaters grows large enough, according to the Jacoby thesis, the social pressure makes a U-turn. The law-abiding motorist becomes the misfitho drives everybody nuts. The horn- honking that used to be a sign of outrage against the light-runner is now aimed at the guy who refuses to run the light, who slows to a stop when he sees a yellow light. "Fool!" the blaring horns scream. "If you accelerate through the yellow, then I can get through the intersection as well. How dare you be so inconsiderate!"

A good example of what happens is reflected in the letters to the editor of The Post. One reader wrote a three- sentence letter in which he said he drives in the fast lane of divided highways but sets his cruise control at 55 miles per hour and refuses to move to the slow lane just because traffic stacks up behind him. "Why should I inconvenience myself for someone who wants to speed?"

That letter ran more than three weeks ago, and outraged responses are still pouring in. The social pressure has shifted, and now it is the law- abider who comes off as a selfish, pigheaded lout.