Unlike front fog lights, which should be turned on as soon as visibility gets poorer, use of the rear fog lights is permitted only when visibility has been reduced to less than fifty meters. This is because the luminosity of the rear fog lights is thirty times more intense than that of the rear lights. For visibility greater than fifty meters, the rear fog lights are very likely to blind motorists following along behind. The reflective guide poles along the motorway are a big help in estimating the visibility range. They are positioned at intervals of fifty meters. Nowadays rear fog lights are mandatory equipment on all vehicles. The lights themselves have been around since 1967. As late as 1970, 3000 accidents were due to poor visibility on account of fog. Twenty years later the German Federal Agency for Highways registered only 1457 such accidents.