order for drivers to read them at night. There are three commonly used grades of materials used: Class 1 (engineering grade) is a low-performance glass bead product, it was the first reflective material used on the UK network and invented by 3M. Today in the UK it is used only for street nameplates and parking signs. Class 2 (high-intensity) is generally a microprismatic product which uses truncated cube corners to return light to the driver. It is commonly used for directional signs or less important regulatory signs. There are also high-intensity glass bead products that meet this class. Class 3 (diamond grade) is a high end microprismatic product for important signs, those on high-speed roads and in areas of ambient lighting where a driver may be distracted by the light clutter. Class 3 can be divided into two divisions; 3A for long distance and 3B for short distance. Currently, the only product that performs to these levels is Diamond Grade DG3. Categorisation Road signs, markings and signals in the United Kingdom are formally categorised into sets, identifiable by the design number: Warning signs (5xx) Regulatory signs (6xx) Level crossing signs (7xx) Information signs (8xx) Bus, tram & cycle signs (9xx) Road markings (1xxx) Direction signs (2xxx) Traffic signals (3xxx) Signals for crossing facilities (4xxx) Lane control signs (5xxx) Road works signs (7xxx) For clarity, signs are more commonly grouped together below Warning signs The importance of a warning sign is emphasised by the red border around its edge and the triangular shape. Some warning signs do not have a red border like sharp deviation. The priority of traffic through the junction is indicated by the broader line. Examples below exclude low bridges, railway and tramway level crossings, bus and pedal cycle facilities, traffic calming and road works. Regulatory signs Signs in circular red borders are prohibitive, whether or not they also have a diagonal red line. Circular blue signs mainly give a positive (mandatory) instruction. Such circular signs may be accompanied by, or placed on, a rectangular plate (information) that provides details of the prohibition or instruction; for example, waiting and loading plates and zone entry signs. 'Stop' signs (octagonal) and 'give way' signs (inverted triangle) are the two notable exceptions, the distinctive shapes being recognisable even if the face is obscured by dirt or snow. Speed limit signs The United Kingdom national speed limit for cars and motorcycles is