What does a blue circle with a number mean?
Minimum speed limit. You must not drive slower than the speed shown unless it is unsafe to do so.
A blue circle with a white number, for example 30.
What you must do
Keep at or above the minimum speed where it is safe.
Where you will see it
Some tunnels and fast roads where slow vehicles cause danger.
Easy to confuse with: Speed limit
A red ring sets a maximum speed; a blue circle sets a minimum. See the speed limit sign.
Test yourself
The theory test asks you to recognise signs like this one. Try the road and traffic signs quiz (with an easy and a hard set), or read the signs revision notes. Source: Know Your Traffic Signs.
Other common road signs
Meaning based on Know Your Traffic Signs and The Highway Code under the Open Government Licence v3.0. The sign image is our own simplified rendering for learning. Always check the current Highway Code on gov.uk. Not affiliated with or endorsed by the DVSA.