1989: Mexico City introduced the Hoy No Circula environmental programme to reduce air pollution. Hoy No Circula prohibited most motor vehicles from the city’s streets on one day a week, based on the last digit of their number plates. Vehicles whose number plates ended in 5 or 6, for example, were banned on Mondays.
This restriction, which was vigorously enforced, applied to 2.3 million vehicles, or 460,000 vehicles on each weekday. Obviously, removing this number of vehicles from circulation cut pollution at a stroke.
Well, actually, no. Many of Mexico City’s car owners, rather than switch to public transport, got round the ban by purchasing a second car – and these cars were generally older and their engines dirtier.
Instead of leading to a decrease in pollution, Hoy No Circula, if anything, produced an increase.
Source: Journal of Political Economy, 2008