MEXICO CITY (AP) — Mexico City authorities doubled the number of cars that must stay off the streets Wednesday as the city headed into a second day of a pollution alert decreed because smog is at 1½ times acceptable limits.

The ban applies to 40 percent of cars in the metropolitan area, or over 2 million cars for the region, which has about 20 million residents.

The city is offering some free bus services and taxi discounts to ease the inconvenience.

Under new rules that began Tuesday and will be in effect through June, generally one-fifth of the cars are idled every day, but the percentage doubles when pollution is 1½ times limits.

Previously, newer or cleaner cars were exempt from the driving ban.

The final digit of a car's license plate numbers determines when it can't be used.