Arlington County Board member Chris Zimmerman, a leading advocate for mass transit and smart growth, said Wednesday that he will step down at the end of January, 10 months before the end of his term.

In a surprise announcement that drew dozens of county employees to the board room, he said he was resigning to take a position as a vice president of Smart Growth America, a District-based nonprofit organization that defines itself as a national coalition of advocacy groups working to promote walkable neighborhoods near public transit.

The county will hold a special election to fill Zimmerman’s seat within 60 days of his departure. An additional election will be held next November for the regular four-year term.

Zimmerman, 54, said Wednesday that he had decided in 2010 that this would be his last term in office and that he devoted the past three years to completing projects in which he had a special interest. The job offer accelerated his plans, he said, but he felt confident that his long-term projects were on stable footing.

Zimmerman accepted the offer 10 days ago but held off on making his decision known until after Tuesday’s election. He said he did not want to draw attention away from the Democratic effort to retake the Virginia governor’s seat.

Serving his fifth term on the board, Zimmerman has been a strong advocate of the Columbia Pike streetcar line and the related preservation of existing affordable housing in the neighborhoods around it.

His accomplishments include establishing the first on-street bike lanes in the area, which now cover 49 miles of the county; starting the local bus service, Arlington Transit, whose 13 routes carry more than 2.5 million passengers a year; saving more than 6,500 committed affordable-housing units county-wide; and helping carry out improvements in the Rosslyn-
Clarendon-Ballston corridor, Pentagon Row, Potomac Yards and Shirlington.

Board colleagues said his new job is a perfect fit for the policy-oriented Zimmerman.

Board Chairman J. Walter Tejada, who with Zimmerman has cast the most liberal votes on the all-Democratic board, called him “an extraordinary public servant.”

Jay Fisette, who was reelected to his fifth term Tuesday, said his colleague “is passionate about these [smart-growth] issues. People that preceded us on the board created the framework. . . . Chris has led the board and the community in refining that framework.”

Zimmerman, a married father of three and a grandfather of one, is so dedicated to the idea of public transit that he routinely takes Metro and buses to his meetings in the area.