I would nominate the completion of Highway 61 along the north shore in the 1930s. First of all, it was a substantial engineering feat, involving blasting tunnels through solid rock, building roadways along steep precipices above Lake Superior and handling the logistics of a major construction project in what was substantially wilderness. It opened up the North Shore to tourism, making Minnesota a major destination for vacationers from throughout the midwest and the rest of the country. In the late 1980s it was named the second most beautiful drive in the country, after coastal route 1A in California. It effectively linked Minnesota to Canada, with benefits for people on both sides of the border. And it was essential in building an economy in a part of the state that -- after the lumbering and mining bottomed out -- had virtually none.
~Doug Wilhide, Minneapolis, MN