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Just as carpenters are told to ''measure twice and cut once," state officials should recheck key assumptions before they go ahead with plans to extend the Green Line to Somerville and Medford.
They should start with claims that the project will greatly reduce air pollution. Air quality was the main reason state environmental officials required construction of 14 transit projects, including the Green Line, when they approved the Big Dig's key environmental permit in 1990, ruling that the projects were ''absolutely necessary to achieve greater air quality improvements in metropolitan Boston."
Eleven of these projects mainly extending and adding commuter rail lines have been built or are under construction. Three are delayed: the Green Line, connecting the Red and Blue Lines at Charles Street, and restoring the Arborway trolley line. Consequently, the Conservation Law Foundation, which pushed for the original transit commitments, and others are suing to force construction of the projects because, in the words of CLF President Philip Warburg, the projects ''help make up for the air pollution generated by the cars and trucks using the Big Dig road system."
The data, however, tell quite a different story. In 1991, while conducting analyses required by the Clean Air Act Amendments of 1990, state environmental and transportation officials found that the 14 transit projects would produce less than 1 percent of the two pollutants that combine in sunlight to form smog: volatile organic compounds and nitrogen oxides. More recently, data in transportation and air quality plans issued by state agencies in 2002 and 2003 show that the Green Line extension would produce a less than .005 percent reduction in emissions of volatile organic compounds and nitrogen oxides in eastern Massachusetts. The Red Line/Blue Line connector and Arborway project would produce even fewer benefits.
The problem is that building the Green Line will have virtually no impact on automobile traffic. Depending on which MBTA study you believe, it will induce between 3,540 and 14,000 people to switch from cars to transit. Even the higher figure is a miniscule share of the 770,000 people who drive to work in Boston or the 1.8 million people in the region who commute by car.
In addition, because of stricter federal standards, cars built since the mid-1990s emit 97 percent fewer volatile organic compounds and 88 percent less nitrogen oxides than cars built in the '60s. As a result, while most cars produce very little pollution, about 10 percent produce roughly 50 percent of all pollution from cars. If the state replaced 500 of these high-polluting cars with Toyota Priuses, it would achieve the same air quality benefits as getting 14,000 people to abandon their cars for the new Green Line. The Prius strategy would cost about $10 million, about 2.5 percent of the Green Line extension's estimated capital cost.
Defenders of the other Big Dig transit projects have responded such data by claiming a narrow focus on air pollution ignores the transit projects' many other benefits, such as spurring compact development and aiding less affluent communities. State officials should assess such claims before they proceed. There is, for example, no clear data showing that regions with extensive downtown-oriented rail transit systems have more jobs in their urban core than comparable areas without rail transit.
The Green Line extension will cost at least $340 million to build; the two other unbuilt Big Transit projects will cost about $250 million. Before we spend that money we should ask whether other transit projects, including other rail transit projects, can produce greater benefits in terms of air quality, congestion reduction, mobility, economic development, and economic opportunity. We should ask, moreover, whether we can achieve the same goals more effectively via careful investments in other areas such as health care, education, the environment, or even tax cuts.
We should not, however, proceed on the basis of unsubstantiated claims of enormous benefits, particularly when some of those claims are not true.