Retroreflective road studs are frequently used to supplement longitudinal road markings. They depend for their brightness on the light from a vehicle's head lamps being reflected back towards the source. The driver sits behind the headlamps and sees a bright reflection from the stud. They are used in addition to reflectorised lines where traffic flows are high, particularly on roads without street lighting. They remain effective in wet weather and also in areas prone to fog, when the efficiency of reflective markings is reduced.
Road studs may be used with most other longitudinal white lines . They are always used on motorways, and are recommended for use on high speed all-purpose roads, even when street lighting is provided.
Studs may be permanent or temporary, and use a glass or plastic reflector, or a plastic reflector with an abrasion resistant protective coating. The standard provides several different photometric performance classes, depending on the stud type.
Studs may also be depressible or non-depressible. In the former, the stud is mounted in a base unit which is embedded into the carriageway. It is designed in such a way that a passing vehicle depresses the stud thus wiping the reflectors clean by a "squeegee" action. Non-depressible studs with rigid retroreflectors can be surface bonded or anchored using suitable adhesives, or embedded into the carriageway using a base unit.