Minimum Speed Limit sign
You may not drive slower than the number shown unless weather, traffic or an emergency makes it unsafe to go that fast.
Federal designation R2-4 in the MUTCD, the national standard every state signs to. Posted on interstates and other high speed divided highways, almost always directly below or beside the maximum speed limit sign for the same stretch.
How to recognise it
This sign is a tall rectangle in white, the standard background for a rule you must obey. 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
This is the rare sign that punishes driving too slowly rather than too fast. On a multi-lane freeway, sitting in the left lane below this number is treated the same as a speeding violation in several states.
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
One Way