The new green construction policy was unanimously approved by the Metro Board and applies to Metro projects that have yet to be built.

The policy provides a set of emissions standards for different types of construction equipment. It also — and I think this is great — has a policy to help regulate the time that equipment sits idling.

It’s a key issue because Metro in the next couple of years is scheduled to embark on some big-time projects, including the Westside Subway Extension, the Crenshaw/LAX Line, Expo Line phase II and the Regional Connector.

There’s a bigger context here: in recent years, there has been a debate big transit projects create more air pollution during their construction than they eliminate once service begins.

Here’s something I wrote about that issue in a post last year about a federal report that looks at greenhouse gases produced by transit:

Studies have found that greenhouse gas emissions from transit are significantly higher when construction of transit is factored into the equation. That’s not good and a lot of hay has been made of that both in academic circles and, in particular, among anti-rail advocates. That said, even with construction as a factor, transit still produces fewer greenhouse gas emissions than private cars — on the condition that transit passenger loads reach certain levels. A subway, for example, needs 19% of its seats filled to be competitive with private cars in terms of lower emissions.

“To the extent that we get this right it will set an example for other industry sectors and our region will be better for it,” said Board Member Mark Ridley-Thomas.