Mexico City has extended into Wednesday a pollution alert that requires 40 percent of cars and trucks to keep off the streets.

Under a rule in effect through June, one-fifth of the city's cars normally must stay at home on a weekday, with the day determined by license plate numbers. But on Tuesday, smog stayed above 1½ times acceptable limits for the second straight day, meaning an additional 20 percent of vehicles can't be used Wednesday.

Before the rule was implemented last month, newer or cleaner cars were exempt from the one-day driving ban.

Air pollution is high due to seasonal weather patterns and increasing number of cars in the city.

Authorities have pledged to come up with new, tougher vehicle emissions tests.