Mexico City residents packed buses and subway trains and many walked or biked to work Wednesday as authorities barred millions of vehicles from the streets due to a pollution alert.

Under new regulations imposed after the capital recently experienced its worst air-quality crisis in over a decade, 40 percent of cars were ordered off the road after smog levels hit 1.5 times acceptable limits on Tuesday.

Pollution levels went down somewhat on Wednesday, and the Environmental Commission of the Megalopolis said only about half that amount of vehicles would be kept off the streets on Thursday.