Laerdal Tunnel

To connect the Norwegian cities of Laerdal and Aurland highway engineers were forced to solve a small problem: the Hornsnipa and Jeronnosi mountains. Instead of going around the obstacles, they decided to go through them. The result was the Laerdal Tunnel, which runs through solid gneiss rock for 15 miles (24 kilometers), earning it the title of the world's longest completed road tunnel.
Excavating such a structure is only one of the challenges. Designers also have to ensure that motorists can make the long, underground trek without succumbing to "highway hypnosis." To address this problem, the Norwegian Public Roads Administration called in a team of psychologists to make sure the finished roadway was as stimulating as possible. The agency recommended including blue lights and gentle curves to keep drivers engaged. They also suggested that the final tunnel be divided into four sections to help reduce monotony.
Motorists entering the Laerdal Tunnel today might not notice these design enhancements, but they'll certainly appreciate them when they emerge safely into daylight after the 20-minute journey through the middle of a mountain.