What is a + 15 (plus fifteen) ?

Calgary has about 18 kilometres of pedestrian walkways with 63 bridges and counting in the +15 system (plus fifteen). This makes Calgary's plus 15 system the largest of its kind in the world. The first +15 bridge was built in the 1970’s linking the Westin Hotel to Calgary Place across 4th Avenue. It soon became known as the "Plus 15 System" because the bridges are located about 15 feet above ground level. Another huge benefit is with Calgary's winter weather the plus 15's make it easy and comfortable to get almost anywhere downtown while staying inside.