“Semi-trailer” shall mean a vehicle of the trailer type so designed and used in conjunction with a self-propelled vehicle that a considerable part of its own weight or that of its load rests upon and is carried by the towing vehicle, and not including a pair or set of wheels commonly used as an implement for other purposes than transportation, provided, however, that the word “semi-trailer” shall not include any vehicle as described in this section if used exclusively for agricultural purposes unless said vehicle be of the type constructed specifically to be towed by a tractor-type truck.