The French have two loves that mix badly -- authomobiles and alcohol. But now, in a revolutionary move, the police here are enforcing a new law that cracks down severely on tipsy motorists.

Hailing the tough measures, which are already producing salutary consequences on the roads, Minister of Justice Alain Peyrefitte recently said: "The French formerly had the right to roll under the table and roll along in their cars. Now they can no longer enjoy both rights simultaneously."

Predictably, the curbs on drinking and driving are opposed by the owners of bars and cafes as well as by elements of the wine industry, who complain that the stiff regulations are killing their business. Not long ago, the head of one wine-growers' association proclaimed the law to be a "violation of civil liberties," and he threatened to resort to "illegal" action against it.

Surprisingly, though, opinion surveys show that the public overwhelmingly approves the effort to improve safety on France's highways. The largest proportion of those in favor are young people.

The new law, which was passed by the French legislature in July, authorizes the police to stop drivers at random and require them to take balloon tests that measure the alcohol content in their blood.

Motorists whose blood contains 0.80 grams or more of alcohol per liter are liable to lose their licenses and may face prison terms of up to four years as well as a fine of nearly $5,000.

The decision to introduce the law stemmed from two factors -- the carnage on French roads and the significant drop in accidents that followed two earlier innovations.

The slaughter broke all records in 1973, when highway fatalaties in France reached 15,500, or one death every 20 minutes. That massacre prompted the government to make the use of seat belts mandatory and to impose speed limits ranging from 54 to 78 miles per hour, depending on the road.

The change was instantaneous. The number of highway deaths declined to 13,500 in 1974 and to 13,170 in 1975 despite an increase in traffic during those two years of about 7 percent. The lack of progress after that, however, led the government to consider rigorous steps to prevent drunken driving.

In studying steps that might be taken, researchers found that only 3 percent of all motorists could be classified as excessive drinkers. Yet they accounted for 40 percent of fatal accidents in daytime and 50 percent of road deaths at night.

The answer, quite obviously, was to discourage driving and drinking. Hence a balloon test of the kind that has long been common in countries like Sweden, where tough regulations have drastically reduced road accidents.

But one problem that has still not been solved is how much liquor is too much. In order to dramatize the problem, the Paris daily newspaper, Le Figaro, invited four people to a meal designed to assay their capacity to absorb alcohol.

The guests consisted of a woman weighing 112 pounds, a skinny man of 141 pounds, an average Frenchman of 154 pounds and a fat fellow who tipped the scales at 194 pounds. Their luncheon, which lasted 90 minutes, featured fish, meat, cheese and dessert. Each guest was requested to drink an aperitif beforehand and two half-bottles of wine, one white and the other red, during the meal.

An hour later, they were conducted to a laboratory for blood tests, and the outcome was astonishing. Only the little lady went over the legal limit with 0.92 grams of alcohol per liter of blood. The fat fellow's alcohol content, in contrast, was only 0.05 grams.

Any physician could have forecast that rich foods lessen the impact of alcohol. The results, therefore, would have been different had the guests simply eaten steak and salad.

But the experiment, by proving that the effects of alcohol on individuals vary widely, has furnished ammunition to those who argue that the present balloon test is too liberal. They assert that the critical level for drinking drivers ought to be lowered to 0.50 grams of alcohol per liter of blood.

In any case, the new law has demonstrated that monitoring motorists can work effectively. As early as August, only a month after the legislation had gone into effect, the highway accident rate had dropped 12.6 percent compared with the same period a year before, and road fatalities were down by 14.2 percent.

Nobody regards it as unusual that special-interest groups, like the liquor business, are squawking. What is unusual, however, is that France as a nation is adapting to a radical regulation that, in times gone by, might have caused culture shock.