What Car? Q&A June 2009 - Temporary speed limit signs
Maria Johnston, via e-mail
A) If the speed is surrounded by a red circle, then it is compulsory and must be obeyed. Go any faster and you risk a fine and points on your driving licence.
Advisory speeds are surrounded by flashing lights. These usually appear in bad weather or heavy traffic. While they aren’t compulsory, we strongly recommend that you stick to them.