All parking areas, service yards or other vehicle areas heretofore or hereafter constructed, which slope down to adjoining properties or streets, or such area which has a drop-off grade separation in relation to adjoining properties or streets, shall have provided by the owner, operator or lessee a wall, sturdy railing, or other installation which will prevent a slow-moving or driverless vehicle from escaping such area. (1961 code § 16.43.010.)