US District Court Judge William G. Young ruled that the city exceeded its legal authority when it imposed the rule intended to convert the entire fleet of 1,825 licensed cabs into hybrid cars.
The city, he wrote, had infringed on the Energy Policy and Conservation Act of 1975, which established fuel economy standards for vehicles and forbids local officials from setting up their own standards.
In an unusual introduction to his ruling, Young noted that his 10-year-old grandson had heard arguments recently in the case and wondered, ``Why can't Boston do what it wants with its taxis? It's for the environment.''
``The answer, Cam, is that the Congress of the United States, pursuing national goals it considers important, has forbidden Boston from taking this initiative on behalf of its citizens.''
The ruling delighted the Boston Taxi Owners Association, which sued the city in federal court, saying the hybrid requirement would cost cab owners thousands of dollars and could put many out of business.
``We think the judge made the right decision,'' said Paul H. Merry, a lawyer for the association. ``The cab operators have wanted from day one to support the city's efforts to protect the environment. But those efforts need to be conducted in a way that do not deprive people of jobs.''
He said he suspected the city will appeal the ruling to the US Court of Appeals for the First Circuit but hoped that the Menino administration will instead sit down with cab drivers and discuss a less onerous way to improve fuel economy for taxis.
William F. Sinnott, the city's corporation counsel, said he needs to meet with the Police Department's Hackney Carriage Unit, which regulates the taxi industry, before he can say whether the city will appeal.
``We appreciate Judge Young's thoughtful consideration of the issue,'' Sinnott said. ``We're reviewing his memorandum and order, and we will meet with our clients shortly and assess our next steps.''
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