Something as pedestrian (sorry--I couldn't help myself) as skyways may not seem to merit consideration as one of the honored 150, but I think their unexpected impact makes a good case for their importance.
It had to do with Minnesota's weather. People going shopping don't relish the idea of leaving their cars. The first skyway, put up in St. Paul in 1956, linked the Golden Rule department store with their parking ramp across the street.
But probably the development of a skyway system, rather than an isolated structure, started in Minneapolis in 1961. The idea was to link up a series of buildings so people could move around without going outdoors. It was one answer to indoor shopping at the malls.
Here's where the impact beyond warm convenience comes in. The new system eventually changed a basic rule of real estate--that first-floor locations went for premium rents. When more people moved through the skyways, stores sprung up on the skyway level and, for the first time in history, second-floor space was more expensive than street level.
~Steve Trimble, St. Paul, MN