This article shall not apply to: The transporting of any child in a vehicle having an interior design which makes the use of such device impractical; or The transporting of children by public transportation, bus, school bus, or farm vehicle. For the purposes of this section, “farm vehicle” means a vehicle which is either (i) exempt from registration pursuant to §§ 46.2-664, 46.2-665, 46.2-666, 46.2-667, 46.2-670, or § 46.2-672, (ii) registered as a farm vehicle pursuant to § 46.2-698, or (iii) owned by a resident of another state under whose laws the vehicle is either registered as a farm vehicle or exempt from registration by virtue of its use as a farm vehicle.