Car ban fails to curb air pollution in Mexico City
Banning cars on Saturdays in Mexico City hasn't reduced air pollutants, according to a new study.
Scientists had expected that limiting driving at the weekend would reduce vehicle emissions by 15%.
But this analysis looking at pollution measurements in a city with serious air quality problems, found no discernible effect.
Residents got round the restrictions by car pooling, using taxis and purchasing extra vehicles, researchers say.
Back in 1992, the UN declared Mexico City the world's most polluted city.
Massive growth in the use of cars coupled with a geographic location that trapped a toxic blanket of dirty air over the city saw tens of thousands of people hospitalised every year.
In an effort to tackle the problem, restrictions were introduced in 1989 with drivers prevented from using their cars on one day per week. The system was based on number plates so a licence ending in a five or six meant the car couldn't be driven on Monday and so on.
The programme, known as Hoy No Circula, has been hugely successful in terms of compliance and has seen some improvements in air quality with Mexico no longer ranked as the most polluted city, having been overtaken on that dubious honour list by the likes of Beijing and Delhi.
Mexico's driving curbs were extended to Saturdays back in 2008 with an analysis carried out beforehand indicating that nitrogen oxides and large particulates would decline by 16%.
To look at the impact of the Saturday restriction, US researchers analysed not just air quality samples but also public transportation numbers and weekend attendance at the city's zoo to get a clearer picture of overall activity.
"I looked at a whole bunch of pollutants, mean levels, maximum levels, every hour of the day, but I couldn't find any evidence that the programme improved air quality," Dr Lucas Davis from the University of California, Berkeley, who carried out the study told BBC News.
"The thinking was it was supposed to get people to take public transportation but if you look at data, they didn't and anecdotally people say they don't take the subway on the day they can't drive, they get a family member to drive them or they take taxis."
Public transport in Mexico City is inexpensive the author says, but often overcrowded. He also believes there are cultural factors behind the reluctance to give up the car.
"Driving is a real status symbol in Mexico City, and once a family have raised enough money to buy a car, there's a status associated with private vehicles that's tough for people to break. There's a bit of a cultural or socio-economic resistance to taking public transport."
Despite this study, other experts believe that Mexico has made significant strides towards improving the environment while both the population and the economy have expanded and hundreds of thousands of new vehicles have come on to the roads.
"Alongside driving restrictions, Mexico City has made massive investments in public transport to provide cleaner alternatives to driving," said Mark Watts, executive director of C40, the global network of cities dedicated to improving the environment and fighting climate change.
"Several new bus rapid transit lines have opened recently and they have the largest year-round bike sharing scheme in North America. At the recent C40 Mayors Summit hosted by Mexico City, Mayor Mancera committed to ban diesel cars from the city by 2025, because they are responsible for the pollutants that are most dangerous to public health."
Madrid, Athens and Paris have have also promised to stop the use of all diesel powered cars and trucks from the middle of the next decade.
Many cities in emerging economies are now putting driving restrictions similar to Mexico in place to curb the growing problem of dirty air. So are there lessons in the Mexican experience that will make the imposition of driving schemes more effective in other growing cities?
"You have to go more directly after pollution," says Dr Davis.
"So that means increasing the cost of driving, and that means higher gas prices, or congestion pricing or parking and it also means more emissions testing and making it more stringent."
The research has been published in the journal, Scientific Reports.