The challenge
Mexico City had some of the worst air pollution in the world, much of it from cars.
The case
In November 1989 Mexico City introduced Hoy No Circula (‘today it doesn’t drive’), banning most vehicles from the roads one weekday each week according to the last digit of their license plate. Economist Lucas Davis studied hourly readings from monitoring stations across the city, comparing pollution just before and just after the rule began. Across pollutants and methods, he found no evidence that air quality improved. Other data showed the total number of vehicles in use grew after the ban, and the fleet shifted toward older, higher-polluting cars, as households bought second cars to drive on their banned day. Some later researchers have questioned parts of the method. A rough calculation reported by Resources for the Future put the restrictions' cost to vehicle owners at over $300 million a year, or $130 per owner.
Why it didn't work
Rules that ration driving by plate number are easy to dodge by buying another car, which can make pollution worse. Pricing or emission standards target the problem more directly.
Sources
- Davis, L.W. (2008). The effect of driving restrictions on air quality in Mexico City. Journal of Political Economy, 116(1), 38–81.
- Resources for the Future. Driving restriction and air quality in Mexico City.
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