New Zealand's Baldwin Street [2] is the world's steepest residential street, according to Guinness World Records, with a slope of 19°.
San Francisco's Filbert Street has a gradient of 17.5° [3] and that's that's steep enough that it has steps.
Of course, a road can be "very steep" and "normal for SF" at the same time! IMHO even if a road is very steep, if you're hiring scooters in a city the brakes (and geometry) should be good enough that an average rider can safely descend the city's steepest roads.
