Calgary's +15 Skywalk system is a network of public walkways suspended at or above 15 feet from street level in the downtown (hence, the +15). It is currently the worlds largest network of such walkways, spanning almost the entire commercial core and connecting residential towers with office buildings, civic centres, and malls.
The +15 network was conceived in the 1960's as one of many ways to enhance the attractiveness of the downtown core to residents and visitors alike. Many other cities at this time such as Montreal, Toronto, and Minneapolis-St. Paul implemented similar system around this time. Interestingly, building the +15 system directly cost taxpayers almost nothing as the network was implemented through provision of the zoning bylaw.
The +15 (as have similar systems across the world) has become controversial in recent years among urbanists and citizens. Some feel it's semi-public, semi-private nature excludes the less fortunate and dampens the vitality of street life below, while others view it as a huge amenity especially in the winter time.
Photo: https://www.google.ca/search?q=%2B15+skywalk+system&rlz=1CAACAG_enCA743CA743&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj464LFzfLTAhVB3GMKHWXFBv0Q_AUICigB&biw=1366&bih=694#imgrc=JQ8X2IH57L9ekM: