A STOP sign was an octagonal traffic sign notifying drivers that they had to stop and give way to other traffic before proceeding.
In 1986, Emmett Brown, who was suffering from amnesia, crashed his steam time car into a sign of this kind at a T-junction in Hill Valley, leaving the post buckled and the sign knocked down.
The towing service Lee Bros. Towing & Repair towed the vehicle to their yard, believing it had been abandoned, leaving a company flyer — which was found by Doc, Marty and Jennifer — attached to the STOP sign (with what appeared to be a piece of chewing gum) to let the owner know where the vehicle was towed.