MEXICO CITY — Mexico City is ordering 40 percent of cars and trucks to stay off the streets Thursday, extending for a third day a traffic cutback aimed at lessening pollution.

Under a rule in effect through June, one-fifth of the city’s vehicles normally must stay at home on a weekday, with the day determined by license plate numbers. But on Wednesday, smog stayed above 11/2 times acceptable limits for a third straight day, meaning an additional 20 percent of vehicles can’t be used Thursday.

Ozone, a key component of smog, reached 1.9 times acceptable limits. The metropolitan environmental commission blamed Mexico City’s typical spring weather: hot, dry weather, a lack of wind and sunny days that favor the creation of ozone.

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

The emergency measures are in effect until June, when the arrival of the rainy season tends to cleanse the city’s air.

Jose Ordonez Diaz, a biology and climate professor at Mexico’s National University, said the double driving ban “has not accomplished anything.”

A smog alert declared last month lasted two days, but the current alert showed no sign of abating in its third day.

“Several institutions have been measuring air quality in recent days with this double driving ban, and there has not been any significant or substantive change detected,” Ordonez Diaz said.

He said the city has underlying problems of poor public transportation, polluting industry and sprawling suburbs.