The world's highest road tunnel opened to traffic in southwest China's Sichuan Province on Tuesday, 15 years after the project was started.

The project, which cuts through Chola Mountain on the Qinghai-Tibet Plateau, consists of a 7-km tunnel and more than 5 km of auxiliary roads.

In Tibetan, the name Chola means mountains so high that eagles cannot fly over them. The peak stands at 6,168 meters above sea level and is covered with snow eight months of the year.

The tunnel is located above 4,000 meters. Its length, the thin air and low temperatures at that altitude all contributed to the complexity of the construction, said Wei Yanqing, chief designer of the tunnel.

A feasibility study started in 2002 and it took eight years for experts to finalize a plan before construction began in 2012.

The project cost about 1.28 billion yuan (196,900 U.S. dollars), according to Wang Qiang, head of the transport bureau in the Tibetan Autonomous Prefecture of Garze, which administrates the project area.

He said the two-way tunnel has a speed limit of 40 kph and takes around 10 minutes to pass through.

Previously vehicles spent more than two hours driving around the mountain on a road threatened by landslides, blizzards and slippery ice, he said.