The motorist had been drinking. But he wasn't driving home. His car, the Chrysler Millennium, wouldn't let him.

The car was equipped with a sobriety test: To start the engine, the driver had to punch a certain set of numbers into the car's computer -- specific numbers in a specific sequence within a specific amount of time. The driver was too blitzed to do it right. He flunked the test and was shipped home in a cab.

Unfortunately, the car in question is as hypothetical as the drunken motorist. It is a "concept vehicle," an idea car, one of 10 on display at the 49th annual National Capital Area International Auto Show, which began yesterday at the Washington Convention Center.

Like most concept cars, the Millennium is headed for the scrap heap, or maybe to the back corners of some automotive research lab where it will gather dust. In either case, it will become one of those "cars Detroit never built," to borrow a phrase from a book of the same title by Edward Janicki.

Indeed, the Millennium already has made the pages of Janicki's book, even though it's still on display. As such, it is a holdover conceptvehicle, one taking its final public bows before being pulled from the spotlight.

Seems funny: Automakers spend millions of dollars developing concept cars and hundreds of thousands more showing them off in elaborate displays worldwide. Then, one day, the dream cars disappear to be replaced by even dreamier models that too will slip into oblivion without ever competing for market share.

What gives?

Research, said Janicki.

"Once built and exhibited, the dream car made it possible to gauge public reaction to its new features through customer research," Janicki wrote. If there is overwhelmingly positive customer reaction to a dream car feature, that feature might show up somewhere on a production automobile. If the public disdains a certain feature, or ignores it, that feature is either trashed or put back on the shelf.

Concept cars have been around since 1938, when General Motors Corp. brought out its first dream-mobile, the "Y-Job." Janicki said that the car had "extras" that no one thought possible then, things such as the now-commonplace electrically powered convertible top and power windows. Dual headlights, automatic transmissions and computerized anti-lock brakes also made their first appearances on concept vehicles.

Automakers never turn out production models that are exact replicas of their concept cars, according to Janicki and others familiar with the ways of Detroit. Development costs, federal regulations, changing markets and myriad other factors work to modify or eliminate the original concept.

Take the Millennium's sobriety test. Though hailed by groups such as Mothers Against Drunk Driving, detractors are telling Chrysler's researchers that the car's test is too tough to pass, drunk or sober. Also, other critics point out that such a device could work against a potential crime victim who, in his flight from an assailant, becomes too frightened and befuddled to properly dial in the car's ignition access code.

However, some features on the Millennium and the auto show's nine other concept cars already have found their way into the marketplace.

Look at the Ford Splash, a whimsical, four-seat, multipurpose vehicle designed by four students at the Center for Creative Design in Detroit. It's painfully obvious that the fluorescent-blue Splash is a youth-mobile. Everything about it screams, "Beach party!" But unlike other expensive concept vehicles this otherwise frivolous buggy is driveable. It is also equipped with some pretty serious stuff -- a sophisticated four-wheel-drive system capable of pulling through sand or snow; a suspension system that automatically lifts or lowers the car, depending on terrain; a built-in roll cage to protect the vehicle's occupants in roll-over accidents; and racing-type four-point seatbelts.

Elements of the Splash's suspension are found in Ford Motor Co.'s Lincoln Continental cars, as well as in the automatic four-wheel-drive systems in some of the company's current cars and trucks.

But four-point seat belts -- H-shaped belts that secure the waist and both shoulders of a vehicle occupant -- might not make it any time soon. Initial consumer reaction is that the belts are ugly and hard to fit over garments like evening gowns, automakers say.

Likewise, many of the concepts in the show's Cadillac Solitaire will remain a dream. The car is equipped with a current socioeconomic no-no, a 430-horsepower, V12 engine. 'Twas a nice idea when Saddam Hussein was a back-page story; but Cadillac officials have already put it on the back burner on a very, very low setting.

Features likely to move from the Solitaire concept car to the new-car showroom include lightweight, high-strength safety glass that could considerably reduce vehicle weight and fuel consumption without compromising safety.

Other ideas on display at the auto show include a computerized navigational system in the Oldsmobile Expression. And then there's the automotive version of the Swiss Army knife, the Pontiac Stinger, which comes equipped with everything from a portable, hand-held vacuum cleaner to a camper stove and fire extinguisher; and the super high-tech Corvette Indy, which has four-wheel steering and automatic traction control, both of which are currently available on some pricey foreign and domestic cars. Rearview vision in the mirrorless Corvette Indy is provided by a dashboard-mounted camera.

In coming years, if Janicki is to be believed, auto shows in Washington and elsewhere are likely to display concept cars that fly.

That's F-L-Y. Yes.

"Cars will fly," Janicki wrote. "They'll become so light that they'll just float on a cushion of air a few inches above the ground, rather than roll on tires."

Hmph. Sort of gives "blowout" a new meaning, doesn't it?