The Federal Highway Administration (FHWA) is concerned about changes in headlamp performance of the current fleet of vehicles in the United States relative to their ability to properly illuminate traffic signs, especially overhead signs. Overhead guide sign luminance, a function of the type of sign sheeting, headlamp illumination, and geometry relative to their locations may be marginal or insufficient for a motorist to read and comprehend the sign in sufficient time to take proper action. A team of Kansas State University researchers was given a contract to determine the minimum luminance requirements for overhead guide signs and to determine if the illuminance from vehicle headlamps on highways was sufficient to provide drivers with this required minimum luminance. This paper covers a literature review to determine the minimum luminance value needed, an overview of the equipment developed for field studies of vehicle headlamp illuminance, the results of field studies to determine illuminance values from a sample of the fleet of vehicles on a US highway, and the results of a study of illuminance values obtained from the headlamps of 50 known vehicles of varying ages and types. Headlamp illuminance readings from samples of vehicles on the highway were taken at points where overhead and shoulder signs would be located. The report discusses minimum luminance values, the illuminance recording equipment, and the conduct of the field studies and results. It concludes with calculations of sign luminance values based on the headlamp illuminance measurements and two assumed types of new retroreflective sheeting material. It was concluded that, even under ideal conditions and assuming that a very low value of sign luminance is acceptable, the fleet of cars on US highways provide insufficient illuminance for this minimum luminance value on overhead and left shoulder mounted signs and marginal luminance for right shoulder mounted signs even when based upon the assumption of new-high performance sign sheeting. If less than ideal environmental conditions were considered, degraded sign sheeting and a less conservative value of minimum luminance were used, the results would be even more critical. How a luminance value should be calculated is discussed. Calculations are presented on hypothetical conditions of degraded sign sheeting materials. (Author/publisher)
Abstract