Seattle’s Sound Transit executed the sale of $477 million in green bonds, which will help fund voter-approved regional transit projects, including work to continue building more than 90 miles of voter-approved light rail extensions.

“Every dollar we save in borrowing costs is one less dollar the taxpayers must pay to build our future transit system,” said Sound Transit CEO Peter Rogoff. “Borrowing at these historically low rates will also insulate us from potential market downturns in the future.”

The bond sale will generate proceeds to fund voter-approved transit expansions. The sale consisted of fixed-rate bonds at a total interest cost of 3.596%. The favorable rate is significantly lower than the 5.3% average over the last 30 years.

The sale represents the second sale of green bonds by Sound Transit, the largest issuer of municipal green bonds in the country.

Green bonds are a rising trend in the financial industry that offers investors the ability to invest in bonds that advance environmental sustainability. Sound Transit’s credibility as a green bond issuer derives in part from its construction and operation of transit services that are more sustainable than other transportation options. The agency observes formalized and independently monitored sustainability commitments across many areas of day-to-day business, from designing energy-efficient buildings to reducing fleet emissions to increasing recycling and composting.

Marketing green bonds is still a relatively new development in the financial sector. Environmental advocates have called for expanding it, with moves toward increasingly rigorous protocols. The ICMA Green Bond Principles are widely considered the prevailing standard. While the green bond designation does not necessarily have a direct impact on the pricing of the bonds, a growing number of issuers use the designation to promote awareness of their projects and expand their investor bases.

Over the past 15 years, Sound Transit has developed a strong track record of delivering mass transit investments. The University Link light rail extension, with new stations on Seattle’s Capitol Hill and at Husky Stadium, open earlier this year, ahead of schedule and approximately $150 million under budget. The extension to Angle Lake opened this September one stop further south from the airport $40 million under budget.