The aim of this study is two-fold: 1) to measure the retroreflectivity of road signs in the field and hence to establish their rate of degradation and the major influences affecting the degradation; 2) to establish a minimum performance criterion of retroreflectivity, a terminal value, below which a sign would become ineffective. Thiswas determined by a literature review, a night-time survey carried out by knowledgeable traffic engineers, and a laboratory experiment.The life performance curves of traffic signs throughout australia have been determined. The minimum luminance required of a traffic sign at night has been found from laboratory experiments to be 3.2 Cd/m2 for all signs other than warning and regulatory signs, where a higher value of 9.7 Cd/m2 is needed. A model for the calculation of terminal value has been proposed. It calculates the minimum distance atwhich a driver needs to see the sign, the total illuminance fallingon the sign, and it uses the minimum luminance required to determine the terminal value of the sign. This will assist the traffic engineer in his decision whether to replace a particular sign, and will allow a 'sensitivity' analysis to be carried out to evaluate the effects of changing variables on the terminal value. The model will alsohelp the selection of the type of sheeting for a new sign. Once theterminal value of a sign in a particular situation has been determined, the life performance data can be used to find the expected lifetime of a particular sheeting material. This will be of benefit in the economic analysis of selecting sheeting materials for signs.
Abstract