Create a stop and go sign without any of the traditional indications of stop and go.
In my visual form class, we were tasked to design a stop and go sign with the above constraint, beginning with the stop sign. I began by sketching fifty possibilities, before narrowing it down to the ten shown below.
From here, I selected option 9 and began iterating on it. 9 was the most effective as the arrow leading to the line conveyed a hard stop, whereas others were slightly harder to distinguish a meaning from.