National highways play a pivotal role in the economic development of a country. Maintaining the road network in optimal condition is imperative to ensure in this regard. An accurate pavement performance prediction model is an important tool for pavement maintenance management to optimize the life cycle cost of maintenance and rehabilitation strategies. At present, there is limited research carried out to develop a pavement performance model for Sri Lanka. In this study, pavement roughness progression of national highways in the long-term performance was analysed to develop an accurate roughness deterioration model. The International Roughness Index (IRI) is a global parameter to measure the ride comfort and the unevenness of pavement. Axle loading and the ageing effect result in functional and structural failure of a pavement which manifests into deterioration of pavement condition. The contribution of different distresses on IRI progression is investigated using the HDM-4 deterioration model. Furthermore, the relationship between IRI progression with pavement age and traffic volume is developed and compared with such models developed in other countries to identify if there exist major differences. The findings of the study could be used in the maintenance management decision making to forecast pavement condition and make future plans.