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Road studs, or cat's eyes, are retroreflective markers on the road surface to supplement longitudinal road markings, aiding visibility at night.
Road studs have a retroreflective surface which reflects the light from vehicle headlights making them visible to drivers in darkness. The colour reflected gives the meaning of the marker (see below).
The studs are constructed as either "depressible" or "non-depressible". Depressible studs have the reflective material on a rubber insert which is mounted in a metal base unit, or "shoe", which is embedded in the road surface. Vehicles driving over the stud depress the insert, the reflective surface of which is thereby wiped clean as it rubs against the lining of the fixed mount.
Non-depressible studs have a reflective surface mounted on a solid plastic or metal body which can either be stuck directly to the road surface or mounted in an embedded metal shoe. Studs mounted directly on the road surface are cheaper and quicker to install, but are more likely to be dislodged by traffic passing over them, so they are normally used only for marking the edge of the carriageway.
Guidance for the correct use of road studs is given in the Traffic Signs Manual, Chapter 5 - road markings.
The original "cat's eye" design was invented by Percy Shaw, who patented it in 1934 and founded the first company manufacturing the products.
White studs are the most commonly seen, they are to indicate the centre line of the carriageway or markings between lanes traffic.
White studs as positioned at 18 m centres when used with centre lines (1005.1) or lane divisions (1008.1) (ie. every two lines), at 9 m centres with hazard lines (1004.1) (ie. between every line) and at 4.5 m centres when used with a system of double white lines.
White studs are uni-directional when used as a lane divider and bi-directional when used as a centre line, except when using in conjunction with a system of double white lines with a hatched centre (diagram 1013.1B) where the studs should be uni-directional so that only the studs nearest to the traffic are visible.
Red studs indicate a line which should not be crossed, this is usually the left-hand edge of the carriageway, but are also used on both sides of a chevron marking bounded by solid lines at a merge or diverge (diagrams 1042 and 1042.1).
Red studs are uni-directional, although red reflectors are often found on the reverse side of amber studs (see below).
Amber studs indicate a line which should not be crossed on the right-hand edge of a carriageway; ie. the central reservation of a dual carriageway road or the right edge of a one-way road.
Amber studs are often bi-directional, with red reflectors on the reverse side which are used when traffic is running in a contra-flow situation as part of temporary traffic management.
Green studs indicate an edge of the carriageway which can be crossed, in conjunction with the edge of carriageway marking (diagram 1010); such as the edge of an acceleration or deceleration lane at a junction or across the edge of a layby. They are also used at lane drop junctions or right turn lanes on dual-carriageways to demarcate the exit lane.
Green studs are always uni-directional and are usually mounted in metal shoes since they are intended to be crossed by traffic.
Temporary studs are used as part of temporary traffic management to mark temporary road layouts. These studs have a fluorescent yellow/green body so they can be seen in daylight as well as a uni-directional amber reflector for use at night.
See also: Raised Pavement Markers - non-reflective markers used instead of markings.