In this reading, the grade-separated pedestrian highway like highways and subways have changed drastically over the years. In the past, these implementation is to ensure the safety of pedestrians crossing the road from the hugh amount of vehicles passing by, creating a more unified retail experience without threatening the primacy of automobiles. In contrast, it had altered the form and spatial logic of cities around the world to a large extent. These kinds of pedestrian highways had changed in the form of footbridges and subways to underground railway systems containing shops over time in the commercial aspect. One of the vivid example is Montreal, the first case of a comprehensive below-grade pedestrian system implemented at an urban scale, integrated with shops and railways, by applying the concepts of urban economics.
What do you think accounts for the changes in the implementation of the grade-separated pedestrian highway over time and space? If you are quoting directly from the reading, use quotation marks.