What is more, European Union countries probably cannot meet a commitment under the Kyoto Protocol to reduce their carbon dioxide emissions unless they curb driving. The United States never ratified that pact.

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European cities also realized they could not meet increasingly strict World Health Organization guidelines for fine-particulate air pollution if cars continued to reign. Many American cities are likewise in “nonattainment” of their Clean Air Act requirements, but that fact “is just accepted here,” said Mr. Kodransky of the New York-based transportation institute.