“New York City has among the dirtiest air in the U.S.,” Ms. Vetter said. “When it comes to ozone and particulate matter, New Yorkers are breathing very unhealthy air. Most of it is tailpipe emissions. And in New York City, where asthma rates are among the highest in the nation, the high ozone levels create very serious threats, especially for kids who spend a lot of time outdoors. Converting cabs from yellow to green would be a great gift to the city’s children.”

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Matt Daus, who heads the taxi commission, which is independent of the mayor, was initially reluctant, but once he learned of the health and other benefits, he joined forces with Messrs. Yassky and Hidary, and the measure passed the City Council by 50 to 0 on June 30, 2005. Since then, more than 500 taxi drivers have converted to hybrids — mostly Ford Escapes, but also Toyota Highlanders and Priuses, and others.

On May 22, Mayor Michael Bloomberg, one of the greenest mayors in America, decided to push even further, insisting on a new rule, which the taxi commission has to approve, that will not just permit but require all cabs — 13,000 in all — to be hybrids or other low-emission vehicles that get at least 30 miles a gallon, within five years.

“When it comes to health and safety and environmental issues, government should be setting standards,” the mayor said. “What you need are leaders who are willing to push for standards that are in society’s long-term interest.” When the citizens see the progress, Mr. Bloomberg added, “then they start to lead.” And this encourages leaders to seek even higher standards.

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I asked Evgeny Freidman, a top New York City fleet operator, how he liked the hybrids: “Absolutely fabulous! We started out with 18, and now we have over 200, mostly Ford Escapes. Now we only put hybrids out there. The drivers are demanding them and the public is demanding them. It has been great economically. With gas prices as they are, the drivers are saving $30 dollars a shift.” He said drivers who were getting 7 to 10 miles a gallon from their Crown Vics were getting 25 to 30 from their hybrids. The cost of shifting to these hybrids, he added, has not been onerous.

Now Mr. Hidary is trying to get law firms and investment banks, which use gas-guzzling Town Cars — 12,000 in the city — to demand hybrid sedans only.

This is how scale change happens. When the Big Apple becomes the Green Apple, and 40 million tourists come through every year and take at least one hybrid cab ride, they’ll go back home and ask their leaders, “Why don’t we have hybrid cabs?”

So if you want to be a green college kid or a green adult, don’t fool yourself: You can change lights. You can change cars. But if you don’t change leaders, your actions are nothing more than an expression of, as Dick Cheney would say, “personal virtue.”