The way they turn out Datsun motor cars in a sprawling plant here does not allow much of a margin for human error. But then, there aren't many humans around to make mistakes.

Inside the body assembly plant, for example, the visitor looks past lines of moving cars, down aisles half the length of a football field, and sees no one. Here and there are small groups of workers watching in case the machines do something wrong. Ocassionally one spots an employe actually touching hands to metal.

But that is an exception. Ninetyseven percent of the work in Nissan Motor Co.'s Zama plant is performed by machines, making it the world's most automated car assembly line. On an average day more than 1,300 cars come rolling off that line. On an average shift, it is manned by exactly 67 workers.

Instead of people, there are robots -- swivelling machines bobbing and weaving as they weld parts together, like so many metallic ducks pecking away at food troughs.

Roofs are welded tight to the body and doors are bolted on without anyone even bothering to watch. Tiny television cameras tell computers when the parts supplies are dwindling and new batches are rolled automatically into place.

American visitors are astonished. Former U.S. Labor secretary William Usery, now a consultant, expressed amazement after a trip through the Zama plant, recently opened to outsiders. "It makes you wonder -- how do we ever compete?" he said.

The Zama plant, 22 miles south of Tokyo, is a battlefront in the coming war for world automobile dominance, a war of precision, uniform quality, low labor costs and, above all, high rates of productivity.

In the past two decades, much of the Japanese superiority in everything from television sets to steel sheets has been attributed to the supposedly happy, docile worker who loyally sings his company's songs and swims contentedly in the company pool in off-hours.

But increasingly, Japan is replacing those cheerful workers with machines or assigning them to watch and repair machines. The result is a steadily rising productivity that is the envy of the industrial world. Fewer people producing more things is the dream of managers everywhere and in Japan they find it closer to reality.

A Western expert in technology was has watched Japan's automation and productivity grow recalls: "Engineers come here from all over the world, and the thing they always say is, "there aren't any people.'"

It is true of autos, steel electronics and just about everything that is made on a large scale in Japan. Usery, for years a labor expert in the U.S., recently visited a Nippon Steel factory. "It was downright scary," he recalled later. "We walked for great distances down those aisles and we didn't see anybody."

The experience has reinforced his judgment that the U.S. has slipped far behind in automation and productivity.

"They are outproducing us in many areas," said Usery, who is a board member of the American Productivity Center. "They (Japanese) spent a lot of money to set this way. We know that their productivity rate exceeds ours." o

Here are a few rudimentary benchmarks on the automation march in Japan:

With the exception of 1974, a recession year, Nissan's plant here has increased production annually for seven years. But the number of employes has not increased at all.

Nissan moved its pickup-truck factory from this site in 1977 to a new location in Kyushu and installed the latest in robotics. The factory now produces the same number of trucks as before with half the employes.

Norman Macrae, an authority on Japanese industry for The Economist magazine, recently resurveyed the state of the art here and compared a Japanese car factory with British Leyland. We found that the proudctivity per man in Japan is four times greater than in British Leyland and it probably will double within the next five years.

Precise productivity comparisions are hard to find. In surveys that compare car manufacturers according to units turned out per worker, Japan turns up with an embarrassingly large lead over both American and European companies. But the comparison is crude and inexact because much of the work done in other nations' factories is done in Japan outside of the plant by subcontractors.

Toyota, Japan's biggest car-maker, devised a formula to measure the amount of value added by each worker in assembly plants. Japan was far ahead of Europe by that gauge but just about even with manufacturers in the United States.

Japan usually is acknowledged to be far in front on one of the major ingredients of productivity: investment in new, automated equipment. It may or may not have a lead in the science of robotics but its application of these inventions is widespread. Faith in the value of new equipment is a cardinal principle.

"Whenever there is money, invest it in machinery," was the rule formulated by Toyota founder Sakichi Toyoda, and most other successful Japanese industrialists have heeded his words.

In the steel industry, for example, Japan makes far greater use of efficient oxygen furnaces than does the U.S. Industry. The result is higher productivity. One 1976 analysis said the average Japanese steelworker produces 400 tons of steel a year while his American counterpart produced only 250 tons. Modernization made steel production 30 percent less costly than in the United States.

Beginning this year, the government will help small and medium-sized businesses make the same kinds of investment in new equipment that the Nissans and Toyotas have made. A special tax depreciation allowance of 13 percent will be granted to the smaller firms which buy robots to replace dangerous and difficult jobs.

The results of Japan's reinvestments are seen in the steadily rising productivity tables. Productivity in manufacturing industries rose 12.3 percent in 1976, 5.1 percent in 1977 and 8 percent in 1978. According to one report, it rose another 12 percent in 1979.

In contrast, the U.S. Labor department reported recently that in 1979 the output per hour of work among major American manufacturers actually fell by 0.4 percent.

Japanese automaters insist that automation in itself is not always automatically a good thing. "The important thing is flexible automation now," said one executive.

That principle is reflected in the Nissan plant here. It is in part an experimental plant seared to the coming world car competition and one example of flexible robotics makes Nissan's point.

It is the newest welding robots which can be programmed by computers to vary their work according to what needs to be done. The old multi-welders were fixed machines which could work only on one model of car.

The distinction is of enormous importance. It means a single Nissan plant in, say, Indonesia could be geared to turn out several models of cars instead of only one, an advantage that may make a crucial difference in who wins that market.