I would have thought Market Street is the longest, although it changes its name from Market to Portola to Junipero Serra and then turns into 280.
http://sflib1.sfpl.org:82/search?/dStreets+M/dstreets+m/1%2C... http://sflib1.sfpl.org:82/search?/dStreets+M/dstreets+m/1%2C... http://sflib1.sfpl.org:82/search?/dStreets+M/dstreets+m/1%2C... http://sflib1.sfpl.org:82/search?/dStreets+M/dstreets+m/1%2C...