The world’s steepest residential road is in New Zealand. Baldwin Street wasn’t intended to be so steep; the town roads were laid out in a grid by planners in London who had no consideration for the terrain and placed it up the side of a hill. At its steepest, the slope has a 35% gradient, meaning for every 9.3 feet you travel, the elevation changes by 3 feet. Source Source 2

Baldwin Street, photographed with the street level: