American reports on overhead signs without external illumination were reviewed, with a view to possible application in the Netherlands. The American situation differs from the Dutch one, in the more frequent use of high beam headlamps, the different characteristics of headlamps, in the larger size of the letters and, possibly in the lower incidence of reduced visibility by fog. The American studies point to a 20% reduction in legibility, for a lone driver with low beam headlamps, when illuminated signs are replaced by non-illuminated signs with an optimal retroreflective sheeting. Calculations were carried out to check for the differences with Dutch circumstances, and as a preparation for experimental verification. Legend luminance was calculated as a function of distance, traffic density and weather conditions.
Abstract