“Skat, husk nu bilen med det rette endetal”

En lille lektie udi utilsigtede konsekvenser for samfundsingeniørerne i Mexico City:

Automobiles are responsible for most of the air pollution observed in Mexico City. In 1989, the government of that city introduced a program, Hoy No Circula (HNC)— English translation: “today it [your car] does not circulate.” The program bans most drivers from using their vehicles one day per week, based on the last digit of the vehicle’s license plate. It was supposed to sharply reduce automobile-caused pollution. In fact, recent research by Lucas Davis (2008) shows that, despite the program’s high costs, HNC failed to produce any improvement in Mexico City’s air quality. [...]

What happened? How could a widely obeyed program that seemingly swept 20 percent of the city’s cars off the streets fail utterly in achieving its goal of improving air quality? The answer is simple: The people of Mexico City bought more cars. A driver with two vehicles can drive every day of the week as long as the last digits of the license plates don’t match days. And because the added cars are only needed 20 percent of the time, purchasers generally did not buy new cars; instead they bought used cars, imported from other parts of Mexico or from the much larger United States market. Older cars tend to be high emitters of pollutants (because they lack the newest emissions control equipment and because such equipment becomes less effective with age). Thus, any decrease in emissions stemming from individuals who chose not to add to their auto inventory was fully offset by the added emissions of the cars purchased to avoid the HNC program.