Onderzoek door Lucas Davis van de University of California, Berkeley had al eerder aangetoond dat overeenkomstig beleid, geïmplementeerd in Mexico City in 1989, de luchtvervuiling niet verminderde – de belangrijkste doelstelling – of de verstopping significant reduceerde.
Research by Lucas Davis of the University of California, Berkeley, had already shown that a similar policy, implemented in Mexico City in 1989, did not reduce air pollution – the intended goal – or significantly ease traffic congestion.