In 2010, Beijing's worsening traffic congestion prompted city planners to move the construction of several lines from the 13th Five Year Plan to the 12th Five Year Plan. This meant Lines 8 (Phase III), , , , the Yanfang line, as well as additional lines to Changping District and Tiantongyuan were to begin construction before 2015. Previously, Lines 3, 12 and 16 were being planned for the more distant future. On December 30, 2010, five suburban lines: Lines 15 (Phase I from to except Wangjing East station), Changping, Fangshan (except Guogongzhuang station), Yizhuang (except Yizhuang railway station), and Daxing, commenced operation. The addition of of track, a nearly 50% increase, made the subway the fourth longest metro in the world. One year later, on December 31, 2011, the subway surpassed the New York City Subway to become the third longest metro in revenue track length with the extension of Line 8 north from the Olympic Green to Huilongguan, the opening of Line 9 in southwest Beijing from Beijing West railway station to (except Fengtai Dongdajie station, which opened on October 12, 2012), the extension of the Fangshan Line to Guogongzhuang, and the extension of Line 15 from to in central Shunyi. In the same year, the Beijing government unveiled an ambitious expansion plan envisioning the subway network to reach a track density of 0.51 km per km2 (0.82 mi per sq. mi.) inside the Fifth Ring Road where residents would on average have to walk to the nearest subway station. Ridership reached 2.18 billion in 2011.