A mathematical approach to define threshold values of pavement characteristics

Transportation authorities managing road networks have to define threshold values of acceptance for pavement surface characteristics. This procedure is either based on relevant experience or drawn from a combination of subjective ranking and monitoring data. Rare are the references of an analytical procedure to define these threshold values with respect to the operational criteria of a road network: safety, comfort, environment and economy. Several road management agencies worldwide apply specific limit values for each pavement feature. Conversely, other authorities do not prescribe acceptance limits at all, allowing for high variability of local factors such as traffic and climatic conditions. In either case, the analytical approach to reliably define limit values is missing. In this study, a methodology to provide threshold values for pavement surface characteristics is presented. Three most important measurable characteristics of pavement conditions, skid resistance, roughness and rutting, are herein analysed. This analysis is carried out by introducing, respectively, suitable parameters, namely the sideway force coefficient, the International Roughness Index and the rutting depth. The objective is to outline a comprehensive methodology for determining threshold values for indicators that portray pavement condition. The proposed methodology attempts to define these limit values by spotting abrupt change in terms of safety or significant increase in negative effects to road users. Relative graphs are given correlating the aforesaid characteristics with safety and travel cost features. The overall analysis is based on the evidence of an inflection' point at each correlation curve, adequately interpreted to provide the threshold values in question.