You have to display, "car in tow" on back window of said car being towed.
Yes it is legal. Law states that the drawbar, chain, rope or other connection between vehicles in combination shall be of sufficient strength to hold the weight of the towed vehicle on any grade where operated and if the drawbar, chain, rope or other connector between the towed and tow vehicle exceeds 15' there must be fastened on such connection in approximately the center thereof a white flag or cloth not less than twelve inches square. Refer to RCW 46.44.070
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