LATIN AMERICA : Future Is Cloudy for Mexico City Smog Law : Study shows that restricting use of cars to certain days does not appear to ease pollution.

It looked simple enough on paper six years ago, when Mexico City’s lawmakers imposed their toughest scheme to clean up some of the worst air on the planet: Every workday, between 5 a.m. and 10 p.m., certain cars were banished from the city’s streets, according to their license numbers.

The “Today Don’t Drive” law was billed as a breakthrough at the time--the city’s first concrete step to cut the auto emissions that were presumed to be a leading cause of the horrific smog that suffocates the Mexican capital most days.

Police won broad new powers to enforce stiff penalties for violators. And, within months of its December, 1989, inception, environmentalists celebrated statistics that showed gasoline consumption down and air quality up in the world’s most populous city.

But now, a new study by Mexico City’s Metropolitan Commission for Pollution Control and Prevention has confirmed what most of the city’s estimated 20 million residents have suspected for years: The law does not work, and it has not worked since soon after it came into force.

In fact, as the city enters the eye-stinging, lung-burning, brown-air days of winter, the commission recommended that the law be either radically modified or altogether repealed. And almost overnight, the study has reopened the debate over the source of--and solution to--the capital’s air-pollution nightmare.

The commission found that the federal district that includes the capital now consumes nearly 2 million more liters--or about 500,000 more gallons--of gasoline each day than it did when the law took effect.

The reason: About 2 million cars were on the streets on any given day in December, 1989; today, there are nearly 3 million.

The result: The city’s air is worse than ever.

Fueled by government policies that encouraged credit financing--and by the new law itself--most families simply bought a second, third or fourth car to get around the restriction, the study said.

The law bans cars one day a week according to the last digit of each license plate. If a plate ends with a 1 or 2, for example, the car may not be driven on Monday; a 3 or 4 bans the car every Tuesday, and so on through 9 and 0 on Friday. By buying additional cars, drivers could acquire a selection of plates that allowed them to drive every day, undermining the system.

And the city’s notoriously corrupt police, rather than enforce the system by the book, used it as a daily cash machine to generate bribes from offenders.

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“Today Don’t Drive has not reduced the number of cars on the street nor increased the use of public transportation,” concluded Luis Manuel Guerra, a leading industrial chemist and air-pollution expert who participated in the study.

What’s more, he said, “if you took all the private cars off the street tomorrow, you would reduce air pollution by only 10%. One of the biggest problems is collective transport--taxis, buses, vans. Also, there is the escape of the hydrocarbons from [liquid petroleum] tanks.”

The study, to be released next week, also concluded that the law failed to take into account human nature in a city where public transportation is cheap but so crowded and inefficient that even the city government has labeled it anarchic.

“It is wrong to think that when the automobile is at rest, everyone uses collective transportation or simply doesn’t travel,” it said. “The facts show that . . . people take taxis or a substitute car, which use the same amount of combustible fuel.”

The chairman of the city’s environment commission said the law will not be repealed until new restrictions are put in place, to avoid a traffic nightmare.

Guerra and the study recommended that, instead of restrictions, the government should create greater incentives for drivers who install pollution-reducing catalytic converters--still a rarity in this city.

And Guerra stressed that the government also must go after businesses that are the worst polluters--a difficult task amid a deep economic crisis that already has hurt Mexican industry and cost more than 1 million jobs.

Yet to be included in the current debate is another recent study indicating that the largest single source of Mexico City smog is not vehicles but liquid petroleum gas leaking from millions of household cooking and heating tanks.