Approximately 20% of the trails in Marin are under the jurisdiction of the Marin Municipal Water District, MMWD. Of the 60 or so miles of hiking trails that are not fire roads, bikes are allowed on 0%. Yes, bikes are not allowed on any 'narrow' trail that MMWD administers. If you'd like to fix this, please feel free to let the Board Of Directors for the MMWD know. They have meetings twice per month.