In this paper, a time-dependent deterioration forecasting model is presented. In the model the deterioration process is described by transition probabilities, which are conditional upon actual in-service duration. The model is formulated by the multistage Weibull hazard model defined by using multiple Weibull hazard functions. The model can be estimated based upon inspection data that are obtained at discrete points in time. The applicability of the model and the estimation methodology presented in this paper are investigated against an empirical data set of highway utilities in the real world.