Mich. Comp. Laws § 257.41

Sec. 41. "Pole-trailer" means every vehicle without motive power designed to be drawn by another vehicle and attached to the towing vehicle by means of a reach, or pole, or by being boomed or otherwise secured to the towing vehicle, and ordinarily used for transporting long or irregular shaped loads such as poles, pipes or structural members capable, generally, of sustaining themselves as beams between the supporting connections.