Photo taken on Jan. 17, 2007 shows the general view of the Zhongnan Mountains Tunnel in northwest China's Shaanxi Province. With a total investment of 3.2 billion yuan, the 18.02-kilometers-long tunnel, the longest in China will open for traffic on Sunday. [Photo: Xinhua]Jan.20 - An 18-kilometer-long, double-tube and four-lane highway tunnel, the longest double-tube highway tunnel in the world, opened to traffic on Saturday in northwest China.The Zhongnanshan highway tunnel, which begins in Qingcha, Xi'an City and ends in Yingpan, Shangluo City in Shaanxi Province, allows vehicles to travel up to 80 kmh.Construction of the tunnel, which ran through the Qinling Mountains in Shaanxi, began in March 2002 at a cost of nearly 3.2 billion yuan (410 million U.S. dollars).The highway tunnel is part of a national highway linking Baotou, north China's Inner Mongolia, to Beihai, south China's Guangxi Zhuang Autonomous Region.Japan is currently home to the world's second longest double-tube highway tunnel, which is 10.9 kilometers.