No person driving or in charge of a motor vehicle shall permit it to set unattended on public, commercial or private property, without first stopping the engine, locking the ignition, removing the key, and effectively setting the brake thereon and, when standing upon any grade, turning the front wheels to the side, such as to the curb or side of a street or highway. A "written warning" shall be given for the first offense on private property, but no such written warning is required for violations on public or commercial property.