Mexico City authorities have declared a pollution alert after smog rose to 1½ times acceptable limits on the eve of an unpopular new program to reduce the number of cars on the road in hopes of improving the capital's dirty air.

A rule that will be in effect through June will keep one-fifth of the city's cars at home every week day beginning Tuesday. Before the rule was decreed last week, newer or cleaner cars were exempt from a regulation that bans cars from driving based on their license plate number.

The city is offering some free bus services and taxi discounts.

Mexico City used to declare pollution alerts after smog reached 1.9 times acceptable limits, or soot reached 1.75. It now declares alerts at 1.5.