Millions of commuters in Mexico City will be scrambling for some alternative means to get to where they're going as the capital city bans millions of older, smoke belching cars to deal with smog that is one and a half times the acceptable limit.

Under rules that took effect on Tuesday, authorities ordered 40 percent of cars to stay home.  This covers more than 2 million personal vehicles in the metropolitan area of some 20 million residents - and it has caused a lot of anger, because the growing middle class is dependent on automobiles for transportation.

But Mexico City is also notoriously under-served by public transportation, and buses and trains are usually dangerously cramped during the peak commuting times - and that's on normal days when all cars are allowed on the roads.  Mexico City Mayor Miguel Angel Mancera asked the federal government for the equivalent of AU$375 Million to improve public transportation - the government of conservative President Enrique Pena Nieto turned him down.

"Given the global economic situation it’s not possible to meet a request of that nature, not for Mexico City or any other state," said Finance Minister Luis Videgaray, suggesting that the city ought to earmark more of its own funds to support public transport.

The new rules will remain in force through June, when the rainy season begins and storms wash away some of the air pollutants.