(a) "Truck trailer" means every motor vehicle designed and used primarily for drawing other vehicles but so constructed as to permit carrying a load in addition to part of the weight of the vehicle and load so drawn.

(b) "Tractor" means every motor vehicle designated and used primarily for drawing other vehicles and not so constructed to carry a load other than a part of the weight of the vehicle and load so drawn.

(c) "Semi-trailer" includes every vehicle without motive power designed to be drawn by a vehicle, motor vehicle, tractor or truck tractor and so constructed that an appreciable part of its weight and that of its load rests upon and is carried by such other vehicle, motor vehicle, tractor or truck tractor. (Ord. 989,, 1990.)