Raised Pavement Markers
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Raised Pavement Markers, informally known as Botts' Dots, are a form of road marking. They were invented in California in 1953 by Doctor Elbert D. Botts who was a scientist working for Caltrans.
They consist of a retro-reflective stud followed by three non-reflective studs. Paint may be laid over the back three studs but this is an optional requirement.
The advantages of the markers is that they not only provide a more visible marker in poor visibility, they also give an audible warning to motorists who have drifted out of their lane (much like the ribbed edge marking in the UK serves to stop people leaving the live carriageway). Disadvantages are that they can be removed by snow clearing operations.
Their use is widespread in the United States, Australia, and New Zealand. There is no bona-fide application of Californian style Raised Pavement Markers in the United Kingdom, although a trial of reflective studs laid out in a similar manner was used on the first sections of the M8 in Glasgow to eliminate the problems posed by continual repainting and burning off of conventional lines during construction. These were removed in 1974 when connecting sections of motorway were opened.
Even today at roadworks in the UK, reflective yellow studs may be used. These are not Botts' Dots although the application is virtually the same. However, the problems posed by HGVs overrunning them and lifting them off the surface have not yet been eliminated.