It consists (in its original form) of two pairs of reflective glass spheres set into a white rubber dome, mounted in a cast iron housing. This is the kind that marks the centre of the road, with one pair of cat's eye showing in each direction. A single-ended form has become widely used in other colours at road margins and as lane dividers. Cat's eyes are particularly valuable in fog and have proven to be remarkably resistant to snow plough damage.
A key feature of the cat's eye is the flexible rubber dome which is occasionally deformed by the passage of traffic. A fixed rubber wiper cleans the surface of the reflectors as they sink below the surface of the road (the base tends to hold water after a shower of rain, making this process even more efficient). The rubber dome is protected from impact damage by metal 'kerbs' - which also give tactile and audible feedback for wandering drivers.
The inventor of cat's eyes was Percy Shaw of Halifax, Yorkshire in England. When the tram-lines were removed in the nearby town of Bradford he realised that he'd been using the polished strips of steel to navigate. The name "cat's eye" comes from Shaw's inspiration for the device: the eyeshine reflecting from the eyes of a cat. In 1934, he patented his invention (patent No. 436,290 and 457,536), and on 15 March 1935, founded Reflecting Roadstuds Limited in Halifax to manufacture the items. The name Catseye was their trademark. The reflective lens had been invented six years earlier for use in advertising signs by Richard Hollins Murray, an accountant from Herefordshire.
James May, co-presenter of the UK car TV show Top Gear said this of this device: "The Catseye is what great design is all about. Simple, functional, and beautiful. And on top of that, this little block of iron and rubber has probably done more to save lives on the road than anything since."
These units are not very visible in daylight and are generally used in conjunction with traditionally painted lines. Temporary cat's eyes with just a reflective strip are often used during motorway repair work and as these are easily visible in daylight as well as in darkness they can be used on their own for lane division.
Also seen during motorway repair work are plastic traffic pillars that are inserted into the socket of a retractable cat's eye rather than being free-standing. These are often used in conjunction with two rows of the temporary cat's eyes to divide traffic moving in opposite directions during motorway roadworks.
Solar powered cat's eyes, showing a red or amber LED to traffic, are being introduced on roads regarded as particularly dangerous. However, shortly after one such installation in Essex in the Autumn of 2006 the BBC reported that the devices, which flash almost imperceptibly at 100 times a second, could possibly set off epileptic fits and the Highways Agency had suspended the programme.
Flashing blue LED cat's eyes were demonstrated on the TV show Accident Black Spot, aired on Channel 4 on 2000-12-19, which alert the driver to potential ice on the road when a low enough temperature, provisionally set at 3 °C, is reached. Proposed enhancements, for an "intelligent cat's eye" of the future, will see the standard white light change to amber for four seconds after the passing of a vehicle, or red if the following vehicle is too close or traffic ahead is stationary.