Buried deep in an attic storage room of the Transportation Department building in Southwest Washington is the bureaucratic approximation of the perfect man. Perfectly average, that is.

Known only as "The 50th Percentile Man," this perfect specimen of positively average proportions has become the prototype for automotive engineers and highway safety experts. He provides, in life-sized display, the federal standards with which all other crash test dummies must comply.

"We have this master model which is sitting here," said Stanley Backaitis, principal engineer at the National Highway Traffic Safety Administration. "There's a pretty good consensus that he's okay."

But after a decade of unquestioned supremacy, The 50th Percentile Man is under challenge from a sleeker, high-technology competitor. General Motors Corp., which helped draft the original specifications for The 50th Percentile Man, is seeking NHTSA approval for its deluxe Hybrid III dummy.

General Motors is not challenging the size or proportions of the old dummy but finds fault with his ability to respond in test crashes of automobiles. Hybrid III, they say, offers more and better opportunities to measure how the human body will respond in various crash situations.

"The Hybrid III reflects to the best of our ability this new information," said C. Thomas Terry, a staff engineer in GM's environmental activities section. "The current dummy has capabilities for eight channels of information. Hybrid III has the capability of 41 channels of information."

GM filed its petition last December to have the Hybrid III accepted as a alternative to The 50th Percentile Man. The NHTSA has accepted the petition, which means that it will be reviewed. A decision will be published in the Federal Register.

The decision involves a review of the federal government's detailed and complex specifications for dummies. These are known to engineers and traffic safety experts as "the 572 rules," for the section of NHTSA's regulations under which they are listed. The specifications deal with detail as intricate as the angle at which the head should pivot when suspended from a pendulum, the distance the neck should rotate and the amount of force the thorax should withstand.

Dummies, called "Anthropomorphic Test Dummies" in the official jargon, must meet two separate sets of specifications, one involving their size and proportions, the other their responses in a simulated crash.

GM's new Hybrid III is about the same size as the dummy stored at the Transportation Department, but the company's engineers said tests on animals and cadavers have enabled them to create "a more lifelike dummy." For example, studies of crushed chests allowed engineers to design a dummy that measures chest injuries more accurately, Terry said. The 50th Percentile Man only measures injuries caused by chest acceleration.

The size specifications aim as closely as possible at the human average. "There is no average person, but what happens is there are statistics taken as to what the average driving population is like, or what the adult population is like," said NHTSA's Backaitis. "The average is not too bad to pick."

The 50th Percentile Man weighs 164 pounds and has a height of 35.7 inches when seated erect. The 50th Percentile Woman weighs 120 pounds and has an erect seated height of 30.9 inches. There are also federally accepted regulations for The 50th Percentile 6-Year-Old child, who is 47.3 pounds and 25.4 inches tall when seated erect.

According to Backaitis, the male dummy if standing would be about 5 feet 8 inches tall. The female counterpart would stand about 5 feet 4.

Backaitis said 100 or so of the vintage dummies will be used when the Federal Aviation Administration runs its simulated crash test of a jetliner at Edwards Air Force Base in California. Some child dummies will also be used.

The American-made dummies, while considered outdated by General Motors, are far more advanced than their European counterparts, according to a GM press spokesman, who said they are often nothing more than "a torso sitting on a bench with stubs for legs."

NHTSA officials said they are not worried about The 50th Percentile Man becoming dwarfed by a taller, broader generation of Americans. "We have kept an eye on the population and what the changes are," Bakaitis said. "Maybe in another 10 years or 15 years, or by the year 2000, there'll be a jump in the average size. But so far, the statistics are fine."