Most commonly, blue signs give mandatory instruction i.e. they tell you something that you MUST do. For example, the sign below tells you that you must proceed in the direction indicated. Blue circular road signs may also indicate which sort of road user can use a particular route, eg cyclists, pedestrians, buses, trams, taxis etc.

The circular sign is informing drivers they must give way to oncoming traffic and the blue sign is informing drivers that they have priority. Road signs for cyclists and cycle lanes are mostly blue, although blue signs may be accompanied with a directional white road sign.