One Way sign
Traffic on this street flows only in the direction of the arrow.
How to recognise it
This sign is a wide rectangle in black, paired with white on regulatory signs. These carry the force of law. Ignore one and you have broken a traffic rule, not just a guideline. Most are white rectangles with black text, and the handful that are not, the stop and yield signs, get their own unmistakable shapes.
What the test wants you to know
Black and white makes it regulatory rather than a guide sign. Turning against the arrow is the mistake that puts new drivers head on into traffic, and it is a standard automatic fail on a road test.
Practise it
Reading about a sign is not the same as recognising it in half a second with an examiner watching. Every state road signs test on this site shows the sign and asks you to name it, then shows four faces and asks you to pick the right one.
Stop
Yield
Do Not Enter
Wrong Way
Speed Limit
No U Turn