20mph speed signs with a green circle?

[spin/trip]

I saw a 20mph speed sign with a green circle round it for the first time the other day. What's the green circle mean?

[Iain]

Minimum

[Connor]

Oh dear.

[Lewis.]

advisory

[Keif]

Green Circled signs are advisory. So it's advising you to drive at 20mph.

Minimum speed signs are white lettering on a blue background.