Quiz - 'Stop' and 'go' designs

Can you tell what the symbols are intended to mean?

The nine pairs of images below, created by undergraduates in lecturer David Reinfurt’s course “Graphic Design: Visual Form,” are meant to convey “stop” and “go” without using existing linguistic or graphic conventions. Which mean “stop” and which mean “go?” Make your best guesses and mouse over the images to reveal the answers.