It's not easy to design a road sign. There are whole books of official guidance on the subject. You have to know a wealth of information to get the right colours, designs, layouts, spacing of text, kerning, tracking and weighting of letters. You need to make sure the radius of curves is correct when compared to the stroke width and letter height. You need to be able to work out when patches are needed and when you're allowed to use a flag or a stack-type sign.
Most of the time the resulting sign is correct, and when British road signs are made properly, they can be great pieces of graphic design work.
Unfortunately it doesn't always go right — whether it's bad design or just a lack of proof reading. Sometimes it's the right sign in the wrong place. This gallery documents some truly terrible mistakes on road signs past and present.