I believe the FAA only recommends taxiing at a brisk walk when in ramp areas. According to the Airplane Flying Handbook, it says to choose a speed that is prudent for the taxiway conditions and allows you to stop in the event of something getting in your way. An FAA bulletin says to taxi at a fast walk when taxiways are contaminated which leads me to believe that taxi speeds when taxiways are not contaminated are above fast walk speeds.