When a road surface is wet, the retroreflectivity of standard glass beads on linemarking is negligible. In recent years it has been demonstrated that large beads, even when the road surface is wet, retain significant retroreflectivity and thus remain visible to drivers. Unfortunately, the larger the beads the more expensive per unit mass they are. How large should the beads be to economically obtain satisfactory wet weather performance ? To answer this question it is necessary to quantify the relationship between the wet weather retroreflectivity of glass beads and their size. This is the aim of the present investigation. The relationship between the glass beads' diameter and wet weather retroreflectivity was measured on a road using a laser retroreflectometer. This was supplemented by laboratory work which looked at the micro-texture of a beaded paint layer. It was found that the larger the beads the better they perform in the wet. To achieve a good wet weather performance, glass beads of about 1 mm in diameter or larger should be used. However, the wet weather retroreflectivity of a beaded paint surface depends not only on the beads' properties but also on how quickly the water drains off, and therefore also on the micro-texture of the surface. (A)
Samenvatting