In 1993, the Federal Highway Administration (FHWA) published research recommendations for minimum retroreflectivity (MR) levels for traffic signs. The recommendations included overhead signs, but not street-name signs. In revisions to the recommended MR levels in 1998, the overhead signs were removed because of unresolved headlamp issues. Since then, there have been changes in U.S. headlamp specifications that prompted FHWA to initiate research to address MR levels for overhead guide signs and, in order to fill a somewhat related void, street-name signs. The intent of the research was to develop recommendations compatible with the 1998 revised recommendations. This report describes the research activities and consequent findings related to the development of MR levels for overhead guide signs and street-name signs. The research included a literature review of the pertinent studies and available photometric models. This review initiated the development of an analytical model to develop MR for overhead guide signs and street-name signs. Using the findings from the literature review and a state-of-the-practice survey, an initial set of MR levels was developed. After an analysis of the initial recommendations, a field investigation was initiated to determine the minimum luminance needed to read overhead guide signs and street-name signs. Special emphasis was devoted to accommodating older drivers. Once the minimum luminance values were determined, the analytical model was used to develop a set of recommendations. The sensitivity of key factors was studied to determine the most appropriate conditions under which to establish MR levels. Once these analyses were completed and the values of the key factors were established, the MR model was executed for the final runs. The initial results are summarised in three tables: one for overhead guide signs, one for post-mounted street-name signs, and another for overhead (mast-arm-mounted or span-wire-mounted) street-name signs. However, these tables were superseded by additional research conducted after this project was terminated. This additional research is summarised and the final recommendations, which were consolidated into one table of MR levels for all white-on-green signs, are presented. (Author/publisher)
Samenvatting