A New Road Roughness Deterioration Model

A new model to forecast roughness as a function of age, traffic, strength and maintenance, under given environmental conditions, has been developed from the World Bank's HDM3 incremental roughness model. This model replaces rutting, cracking, patching and potholing with variables constructed from maintenance records. The surface defects have been dropped because good maintenance practice in the study area prevents ruts, cracks, and potholes from developing so far as to critically affect roughness. In the new model, the modified structure number in the traffic-related term is replaced by falling weight deflection. For this non-linear model, a maximum likelihood procedure has been used to calibrate the parameters. Statistical tests indicate that the estimates have a high level of reliability. The model estimates imply that the age-environment variable explains about 52.5 percent of the roughness changes that would have occurred without maintenance, while traffic explains about 47.5 percent. This result indicates a much greater contribution of traffic to deterioration than has been found in a number of previous studies, partly because the relatively thin pavements in the study area are easily damaged by heavy trucks and also because maintenance effects are specified separately.