The visual information provided by road signs gives the motorist fundamental instructions governing the selection of routes, safety at intersections, warning of physical obstructions, and delineation of a safe path for travel. Signs are the primary means of communication from the road authorities to the motorist about roadway conditions. Signs must govern safe and comfortable travel both in daytime and at night. Official traffic control devices are made visible at night by providing a retroreflective surface, retroreflective buttons, or, in some cases by separate illumination. This preserves the essential visual cues necessary for night-time operation: the discrimination of the sign from the night-time surroundings by its unique colour, shape and legend. Once many such devices have been installed and have been in service for a number of years, degradation of this important system will occur. A continual maintenance effort by authorities is required to ensure timely replacement. Although replacement of stolen signs, vandalised signs or those rendered obsolete by changing regulation is accomplished from reports by police and by routine daytime inspections, the loss of night-time visual performance from the effects of natural weathering is often difficult to discern in daytime and eventually will lead to the gradual degradation of the remaining signs. The night-time effectiveness may then become seriously impaired. This report covers those retroreflective signs dependent on headlamp illumination and provides information on the replacement point for retroreflective signs degraded from the effects of weathering. Because of the very large number of retroreflective road signs and the driver's dependence on their functionality, research has identified a number of factors concerning the maintenance of the visual effectiveness of retroreflective signs. The research includes a number of important elements: the determination of the minimum retroreflectance which can provide an adequately functional level of night-time performance; the prediction of this end point to provide a means of scheduling replacement; means of inspection and providing an inventory; and, ultimately, a systematic method for obtaining the maximum value for each annual expenditure. The principles described in this report may be applied to other signing applications such as signs employed on railways and waterways.