The 55-mph speed limit has been good to the people who run Cincinnati Microwave Inc.

In 10 years, that limit -- and drivers' resistance to it -- has propelled the firm from a humble workshop to the top of a $400 million industry.

CM makes radar detectors, microwave receivers often used by drivers to evade radar-equipped police "speed traps."

The company's business is booming, up 46 percent in sales last year. Declining fuel prices and rising public demand for high-performance cars are expected to keep CM's sales growing by as much as 20 percent annually for the foreseeable future, according to Wall Street analysts who follow company.

At least 14 other companies, many of them small and privately held, also are cashing in on what the government says is the propensity of 51 percent of America's licensed drivers to exceed the 55-mph limit.

An estimated 470,000 radar detectors were sold in 1980, four years after the federal speed law went into effect. Some 1.7 million detectors were sold in 1985, according to radar detector industry figures confirmed by the Department of Transportation.

But criticism of the detectors and the industry that makes them is growing, too.

Companies such as CM "have blood on their hands" because they contribute to traffic deaths and injuries by encouraging speeding, said Joan Claybrook, a former administrator of the National Highway Traffic Safety Administration.

"There is no purpose for radar detectors other than to violate the national speed law. And that law serves a purpose -- it saves lives," said Claybrook, who now serves as president of Public Citizen, a public policy research group based in Washington.

CM executives and others in the radar detector industry call those charges nonsense. Helping people to break the law "certainly doesn't drive our business," said John A. O'Steen, CM's president and chief operating officer.

"We are a legitimate business," O'Steen said. "Our goal is to strive to produce the best products that we can and to provide top-notch customer service. If we've been successful, it's because we've done exactly that. Our customer satisfaction level is probably second to none," O'Steen said.

At issue is the public's right to receive radio waves, a right that was sanctioned by the Communications Act of 1934, said Janice Lee, director of the Radar Association Defending Airwave Rights (RADAR), the radar-detector industry's lobbying and public relations arm based in Ohio.

The FCC-approved X-band (10.525 gigahertz) and K-band (24.150 GHz) that police use to to transmit radar signals are public air waves, Lee said. Radar detectors are no more than radios that are capable of picking up those waves, she said.

"We don't believe radios are illegal," Lee said. "The public has a right to receive those waves. Everyone has a right to know when he is under surveillance," she said.

About 4 million U.S. drivers today are using radar detectors, "and most of those people are not speeders" or reckless drivers, Lee said.

"The people we've polled say that the radar detectors help them to become more aware of their surroundings, and that often makes them slower and more careful drivers," Lee said.

That logic was disputed in April at the 83rd annual meeting of the American Automobile Association, the nation's largest group of owners of private vehicles. The nearly 1,000 delegates and officials attending the AAA conference approved a resolution opposing the use of radar detectors "by all highway users to avoid speeding violations."

"Our concern is that there is a growing level of sophistication in radar detection systems," said AAA spokesman W. Allan Wilbur. Even though no established manufacturer of so-called "passive" detectors sells radar-jamming devices, "you can now buy parts to install a system" to block the transmission of police radar signals, Wilbur said.

CM and other radar detector industry officials say they oppose the production and use of "jamming" devices that can block police radar or force police radar to misread the speed of oncoming vehicles.

"Jammers are not legal, and everybody in this business knows that," said Paul M. Allen, CM's senior vice president of marketing. "There are a lot of jammer products out there that don't work anyway. And if it doesn't work, and it's illegal, what are you going to do? Call the cops to complain? Call Ralph Nader?" Allen asked.

But receiver-model radar detectors are becoming more sophisticated, Allen said. CM's newest detector, the Passport, is a case in point. It is tiny enough to be carried in a shirt pocket, a purse or a briefcase. It can be placed discreetly on a sun visor. Yet, using miniaturized, surface-mounted electronic components, it can pick up and identify police radar signals thousands of feet away from the transmitting source -- in ample time to "inform" or "warn" its user of nearby radar.

Claybrook and other safety advocates argue that radar detector users often slow down abruptly when they pick up police radar. That sudden deceleration can lead to accidents, Claybrook said.

But highway safety experts who support the use of radar detectors say Claybrook is wrong. "It's very curious that someone like Joan Claybrook, who advocates highway safety, never bothered to learn about it from an academic viewpoint," said John Tomerlin, highway safety editor for Road & Track, a national auto magazine.

"The only thing that has a direct effect on traffic behavior is a visible patrol car. If it is not visible, it has no effect. To the extent that radar detectors make patrol cars 'visible,' that makes traffic safer, not more dangerous," Tomerlin said.

The AAA's Wilbur said he is not convinced; but he acknowledged that many of his organization's 26.3 million members in the United States and Canada probably own radar detectors.

"We must be on the side of safety," Wilbur said. "We still support the 55-mph speed limit on a national basis, and the radar detector is a tool for breaking that law," he said.

Most of the nation's state legislatures apparently do not share that view.

There is no federal law forbidding the manufacture, sale or use of radar detectors, so regulation of the devices is essentially a state concern. Of the 50 states, only two -- Virginia and Connecticut, along with the District of Columbia -- now have laws prohibiting the use of radar detectors in or on motorized vehicles.

A RADAR-mounted lobbying campaign almost overturned the Connecticut statute this year. The industry-sponsored bill to legalize detectors passed the State senate but failed by one vote in the Connecticut house.

No similar RADAR effort has been launched in Virginia or the District of Columbia. "We want to get Connecticut first," where the industry has invested much money and time and believes it has a chance of winning, Lee said.

The stakes are high.

CM is by far the largest player in the field. The company had net sales of $112.6 million last year, a 46 percent increase over revenue of $76.9 million in 1984.

CM, incorporated in 1976, two years after the federal speed limit took effect, holds 38 percent of the U.S. radar detector market. That is more than twice the market share of its closest competitor, Controlonics Inc., based in Massachusetts.

Besides the Passport, CM also sells a larger Escort detector. Controlonics, which produces Whistler detectors, holds 15 percent of the market. Electrolert Inc., an Ohio maker of the Fuzzbuster, is third with nearly 13 percent, according to RADAR figures.

"We always said in the early days that if we could ever get 5 percent of this market, that would be amazing," said Paul M. Allen, CM's senior vice president for marketing.

The people who started the company "were just a bunch of young guys," tinkerers and car buffs, who thought they could build a better detector than the basically crude models that were on the market when CM opened shop, Allen said.

"Nobody went into this business to get rich," said the 30-year-old CM vice president. But the company built a better detector, which outperformed competitors in defining police radar signals, Allen said.

The first CM detector, the 1978 Escort, cost $245 -- almost double the cost of the best-selling detectors at that time.

"If you had asked someone then, 'What kind of a market exists for a $245 radar detector?' the answer would have come back, 'None,' " Allen said.

But CM officials said they built the super-detector anyway, partly because of tinkerer's ego and partly because they figured they could at least break even through direct sales of their product in a limited market.

Word-of-mouth advertising built up a clientele for the then-esoteric Escort. And the word went forth with gusto in 1979 when a national car buff magazine, Car and Driver, rated the Escort as the best radar detector on the market.

The CM detectors have been winning comparison tests since then. Wall Street has taken note.

"From its inception . . . through the latter part of 1984, Cincinnati Microwave derived superior growth from the successful development and marketing of its premium quality automotive radar warning detector, the Escort," said John C. Ball, an analyst with PaineWebber Inc., based in New York.

The introduction of the miniaturized Passport "provided the fuel for a continuation of this growth in 1985," Ball said of CM's performance.

Still, some analysts have tended to undervalue CM's stock because they feared that the company "produced only one type of product, which was not only faddish in nature but also highly likely to be universally outlawed through state legislative action," Ball said in his CM assessment earlier this year.

But the radar detector phenomenon "is hardly a fad," Ball said, with sales of radar-detection equipment "growing annually for almost 20 years."

But what if Congress, under growing pressure from motorists and restive state legislatures, increases the 55-mph limit? Eight states, mostly in the West, already have laws that significantly weaken penalties for violating the federal speed statute. And so far, there are three proposals in the House and one in the Senate calling for liberalization of the national speed law under certain circumstances.

"I don't think the market for radar detectors would go away" if such a law passed, said CM's O'Steen. "It wouldn't put us out of business by any means. In the short-term, a period where people would be wondering what this was going to mean, you might see a little drop in sales," O'Steen said.

But he conceded that CM's longterm planning is not to rely totally on radar detector sales. CM is using its ample cash reserves to diversify.

Last June the company paid $5.2 million in cash and shares to acquire Wilson Microwave Systems Inc., a Nevada manufacturer and distributor of satellite television systems. Earlier this year, CM also took over Guardian Interlock Corp., maker of an automobile ignition-locking system designed to prevent inebriated drivers from starting their cars.

CM is planning to market the Guardian device to judicial systems, which would require people convicted of drunk driving to install the mechanisms on their vehicles.

Some judges initially were a bit squeamish about discussing the Guardian Interlock proposal with a company that made its money from selling radar detectors, O'Steen said. "But because we do a quality job of what we do," some local judges were willing to talk to CM, O'Steen said. "They believe that we're going to back up the Guardian system and support it in a way that it ought to be supported."

Is that the corporate equivalent of playing both sides of the street -- on the one side, selling a product that critics say promotes lawlessness, while on the other side, selling something else that encourages obedience to the law?

"No," said O'Steen. "We believe in responsible driving," which is why CM opposes the production of radar-jamming devices, he said.

"Our target is to be the last company involved in the radar detector business, and I think we can do that," O'Steen said. "At some point," with or without restrictive laws, "radar detector sales will peak out."

"None of us would expect the radar detector business to continue growing at 20 percent or 30 percent a year forever. At some point, it's going to level off," O'Steen said.

"But we're going to be ahead of that. We're going to have other businesses supporting us overall and allowing us to move ahead."