Retro-reflective road studs are frequently used to supplement longitudinal road markings. They depend for their brightness on the light from a vehicle's headlamps 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. Studs may be permanent or temporary, and use glass or plastic reflector, or a plastic reflector with an abrasion resistant protective coating or a stud may be of Aluminum. The standard provides seve ra l d i f fe rent photomet r i c performance classes, depending on the stud type.