The World's 18 Strangest Roadways: Gallery

The most direct path between two points is a straight line, but roads are rarely straight, and the ones that are can be terminally boring. Engineers around the world must calculate the most efficient routes over massive mountains, through densely populated cities and around unavoidable bodies of water, all while accounting for the ecological and financial cost of such projects. The results can be astonishing. Here are some of the world's most notable roads and why they stand out.

1: Hana Highway

1: Hana Highway

Maui, Hawaii

Background: Hawaii's Hana Highway runs along the northeast coast of Maui, and is constantly cited as one of the world's most scenic drives.

How It's Unique: The Hana Highway takes drivers around 600 curves and over 54 bridges (most of which are one-way) in just 52 miles. "Whenever you have to build a road like this in a coastal area, your best bet is to hug the terrain," says Hani Mahmassani, a civil engineer who has led more than 100 transportation projects and is the director of Northwestern University's Transportation Center.Â "If you wanted to put something faster-running through this area, you would have to cut through the rock or put in a tunnel, which is very expensive." The road's winding design, narrow passages and stunning views take some slow-going motorists as long as 4 hours to navigate, according to Hawaii's Visitors and Conventions Bureau.

Photograph courtesy of Library of Congress's Prints and Photographs Division

2: Passo dello Stelvio

2: Passo dello Stelvio

Background: This mountain pass in the Italian Alps starts an elevation of 3116 feet and ascends to just under 9050 feet, or 1.7 miles above sea level. The pass was first constructed in 1820.

How It's Unique: The Stelvio Pass has a dizzying 48 hairpin turns and an average grade of 7.4 percent. "To go down that slope, you either basically cut down the mountain to remove that slope, or you have to go back and forth and zigzag a lot," Mahmassani says. The road, which hosts one of the more gruelling stages of the Giro d'Italia bicycle race, has a 7.4 percent grade--"huge" by American standards, Mahmassani says. Six percent grades are about the maximum for U.S. highways, he says, and require escape lanes and safety signage. "You go down or up this road, you're going to get seasick," he says.