In the Seattle metro region, voters just sank an $18 billion transportation megaproposal that would have built more than 180 lanes miles of highway and 50 miles of light rail. But so far, the mainstream press has missed one of the most important stories of the year. The real story isn’t tax fatigue, it’s this: perhaps for the first time ever in the U.S., a critical bloc of voters linked transportation choices to climate protection.

In the run-up to the vote, a surprising amount of the debate centered on the package’s climate implications. (The state has committed to reduce greenhouse-gas emissions to 50 percent below 1990 levels by 2050, and many cities, including Seattle, have been national leaders on climate.)

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The opposition argued global warming. So did the measure’s supporters. If you don’t believe me, see, among others, the Seattle P-I (yes), The Stranger (no), the Yes Campaign, the Sierra Club’s No Campaign, the right-leaning Washington Policy Center (no), and even the anti-tax/rail No Campaign, which oddly enough kept trumpeting the Sierra Club’s opposition as a primary reason to vote no.

The turning point may have been when King County Executive Ron Sims suddenly withdrew his support. He cited the climate-warming emissions from added traffic as one of his chief objections — he was thinking about his granddaughters, he said, not just the next five years.

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The funny thing was, there was a heap of confusion and disagreement over the proposal’s true climate impacts, mainly because no one had conducted a full climate assessment of the measure. But climate clearly weighed as a factor for a critical bloc of voters on both sides of the issue. In fact, Prop 1 may be the last of its kind, at least in the Pacific Northwest: a transportation proposal that lacked a climate accounting.

Obviously, there were more factors in play than just the climate. Taxes and traffic congestion mattered too. But what ultimately may have tipped that scales is that Puget Sound voters are reluctant to expand roads because they lock us into decades of increased climate pollution.

It’s pretty well accepted that Seattle-area voters are receptive to environmental messages — and in this case there were smart and well-informed greens on both sides of the debate. But green or not, the biggest problem for a certain segment of voters may have been that there was no comprehensive accounting of the climate impacts of the project — one that included the roads, the rail, and the probable effects on land use.

So what’s the lesson?

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First, driving is the single largest source of Washington’s emissions, as it is for much of the United States. So, future transportation packages should at least attempt to tally the climate impacts. And as voters begin to take climate protection more and more seriously, transportation packages should address climate change as a guiding principle.

Transportation projects that embrace climate protection will:

  • Stay ahead of the curve by estimating the climate impacts in advance.

    Emissions estimates should be calculated for every large transportation project. Greenhouse-gas accounting is still a new field, so analysts may at first be able to obtain only ballpark figures for the expected emissions from some new projects. But reasonable estimates are still useful. And many aspects of climate accounting are fairly straightforward: we already forecast how many cars a new highway will carry, so why not estimate how much gas those cars will burn?

    The Sightline Institute (where I work) developed a general estimate showing that in congested urban areas a single new lane mile of road adds at least 100,000 tons of greenhouse gases over 50 years. Detailed analyses of direct impacts, especially tailored for local areas, can help planners and voters determine the most responsible solutions for the region.

  • Focus on transportation solutions that are both cost-effective and climate-friendly.

    Smart, small-caliber solutions can improve mobility even as they curb fossil fuel use, and at modest cost to taxpayers. Solutions like boosting ridesharing, speeding bus service, and expanding bicycle facilities, as well as using existing roadways more efficiently through policies like congestion pricing, are cost-effective solutions that can address the region’s transportation challenges while reducing climate impacts.

  • Use the opportunity to improve land-use patterns.

    Our transportation choices and land use patterns are closely intertwined. Adding new highways can induce low-density sprawl, which in turn lengthens trip distances and requires car travel for nearly all trips. New roads can tilt development patterns toward car-dependent sprawl for decades to come.

    Planners should begin to examine the greenhouse gas impacts of building and operating a light rail, implementing HOV/HOT lanes, or fostering compact development near transit. In addition, we should study how adding lanes on the urban fringe may lead to new low-density development and increased emissions.

Puget Sound’s roads and transit measure may have been the last of a dying breed: a transportation package presented to the voters without a clear accounting of climate impact.

Over the long haul, transportation is the most important piece of this region’s climate change puzzle. Analyzing the impacts of our transportation projects can create real opportunities to move climate protection in the right direction.