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In 1989, the government of Mexico City introduced a program, Hoy No Circula, which bans most drivers from using their vehicles one workday per week based on the last digit of the ve- hicle’s license plate. The program has been strictly enforced and has been since emulated in Bogota, Santiago and S˜ao Paolo. This paper measures the effect of the driving restrictions on air quality using high-frequency measures from monitoring stations. Across pollutants and spec- ifications there is no evidence that the program has improved air quality. The policy has caused a relative increase in air pollution during weekends and weekday hours when the restrictions are not. | The Effect of Driving Restrictions on Air Quality in Mexico City Lucas W. Davis University of Michigan December 9 2007 Abstract In 1989 the government of Mexico City introduced a program Hoy No Circula which bans most drivers from using their vehicles one workday per week based on the last digit of the vehicle s license plate. The program has been strictly enforced and has been since emulated in Bogota Santiago and Sao Paolo. This paper measures the effect of the driving restrictions on air quality using high-frequency measures from monitoring stations. Across pollutants and specifications there is no evidence that the program has improved air quality. The policy has caused a relative increase in air pollution during weekends and weekday hours when the restrictions are not in place but there is no evidence of an absolute improvement in air quality during any period of the week for any pollutant. Evidence from additional sources indicates that HNC led to an increase in the total number of vehicles in circulation as well as a change in composition toward high-emissions vehicles. Department of Economics University of Michigan 611 Tappan Street Ann Arbor MI 48109 email lwdavis@umich.edu. I am grateful to Michael Greenstone Steven Haider Sandy Sillman Jeff Smith Gary Solon Miguel Urquiola Dean Yang and seminar participants at Michigan HEC Montreal the NBER Summer Institute Michigan State the UC Berkeley Energy Institute and Harvard for their helpful comments. 1 Introduction Whereas U.S. cities have seen dramatic improvements in air quality over the last three decades1 Mexico City has been considerably less successful. Levels of major air pollutants in Mexico City routinely exceed maximum exposure limits established by the World Health Organization WHO . For example the WHO has warned that eight-hour average ozone levels exceeding 100 micrograms per cubic meter threaten human health.2 During the period 1986-2005 this guideline was exceeded in Mexico City for 92 of all .