Question Topic: Road and traffic signs
Explanation: Road signs in the shape of a circle give orders. Those with a red circle are mostly prohibitive. The 'stop' sign is octagonal to give it greater prominence. Signs giving orders must always be obeyed.
Category: Road and traffic signs
References: The Official DVSA Guide to Driving: Section 6, Highway Code: Page 106, Know Your Traffic Signs: Page 9, Know Your Traffic Signs: Page 16