San Francisco is well-known for its steep streets. The two steepest streets in San Francisco are Filbert Street, between Hyde and Leavenworth Streets, and 22nd Street, between Church and Vicksburg Streets. They have grades of 31.5%, which would mean for every 100 feet one would travel in the horizontal, there would be an increase of 31.5 feet in the vertical. Both of these streets are one-way, going downhill.

However, there are numerous streets in the United States (and the world) which are even steeper than these two legendary San Francisco streets. At best, the San Francisco streets would be tied for #8, and that's if four-wheel-drive roads are excluded (like one in Hawaii that has a reported grade that briefly is as high as 45%), and there are no other streets with grades steeper than 31.5% that I have not listed (unlikely, since these are just the ones I found).

Dunedin, New Zealand claims to have the steepest street in the world (Baldwin Street). However, recent measurements have found the grade to be 35%, not 38% as had been previously thought. Thus, Canton Avenue in Pittsburgh, Pennsylvania (Beechview) could make a claim with its 37% grade. Honokaa-Waipio Road, near Waipio, Hawaii, could also make a claim with its reported (albeit brief) 45% grade, but traffic on that road is restricted to four-wheel-drive vehicles, despite being paved, so it's not the same type of "thrill-ride" that one can take in a normal passenger car.

Note that most of the steepest streets are older streets; many cities have restricted the grades in recent years (for safety reasons; cars with poor brakes, including parking brakes, sometimes crash on these streets). In most steep areas, streets curve up the hill instead of going straight up!