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.

On any given day a fifth of all cars are supposed to stay home, but that doubles during a Phase 1 alert. Until recently, newer and cleaner cars were exempt from the driving ban.

With 2 million of the capital's cars ordered out of circulation on Wednesday, fares were temporarily waved for some public transportation. Buses crawling along the central Reforma Boulevard were crowded during the morning rush hour.

"Public transport is not sufficient," said Martin Colin, a 53-year-old taxi driver, pointing to a bus jammed front-to-back with standing commuters. "They look like little sardines. The buses are full, full."

The vehicle ban does not apply to public or private buses - something that has drawn criticism since many privately operated minibuses are notorious for belching thick black smoke.

A truck drivers' protest against the "no circulation" rule partially blocked two major highways on the periphery of Mexico City, snarling traffic. The truckers are upset that the rule keeps them from making deliveries.

Taxis prowled the streets in visibly greater numbers, but even so, finding an unoccupied cab was more difficult than usual. Dispatchers reported increased demand, and Uber users complained of prices several times higher than normal.

Many residents turned to Mexico City's expanding bicycle-share program for the commute.

Luis Arturo Garcia had to leave his car at home and was inspired to sign up for the bike-share program after his usual 10-minute commute stretched to 30. "It affects me a lot because I work in the morning and study in the afternoon," he said. "They're pretty long trips, and public transportation doesn't work for me."