Usually round. If they stop you doing something, they normally have black symbols on a white background in a red border.
If they demand that you do something, they're usually white on blue with a white border
A diagonal red line through something means NOT-don't even go there!
Some signs are qualified by plates. Exceptions include 'No Entry' (red background) 'Stop' (octagonal), 'Give Way' (upside down triangle), 'Stop Children' (yellow background), National Speed Limit' (white with no border)
In a road with lamp posts the usual speed limit is 30mph. If the limit is 40 or 50, or the national speed limit for that type of road (usually 60mph) applies, small versions of these signs will appear along it.
SIGNS GIVING WARNINGS
Usually these have black symbols on white in a red triangle. If theres no symbol for the hazard, it will be worded in the triangle or on a plate. Signs meaning 'School' may have amber lights which flash when the crossing patrol is there.
Where there's a diagram of a junction, the thick line has priority.
Exceptions include 'Distance to Stop/Give Way Line' (inverted empty triangle), 'Sharp Deviation' (chevrons on black rectangle) 'Level Crossing Without barrier' (red diagonal cross).
SIGNS GIVING INFORMATION
Rectangles or pointed rectangles. Direction signs are white in black border when local or on non-primary routes, green in white on primaries and blue in white on motorways.
Some local information and signs for pedestrians or cyclists (e.g 'Hospital' 'Library', 'No through road') also use blue in a white border. A junction of important roads normally has three signs: one well before it, one after it to tell you where you went wrong. Tourist information signs are brown, and temporary signs such as diversions and contraflows tend to be yellow.
