The estimation of the costs of building rural dual-carriageway roads has been studied and a method has been developed in which the costs of earthworks, drainage and culverts, pavement, bridges and preliminaries and miscellaneous items are calculated seperately and aggregated. This method has been incorporated in a computer program, COSMOS, which has been tested on several major highway schemes and used by designers in Road Construction Units and in local authorities. It is suitable for budgetary purposes and for costing alternative alignments. COSMOS has been found to produce estimates which are more accurate than those obtained by current methods and to be convenient to use. It shows the engineering assumptions made in the estimating process so that doubtful results can be examined. The sensitivity of COSMOS to uncertain cost data and to incomplete data on the design of a scheme has been studied. COSMOS costs about Å“2 per run through the Highway Engineering Computerb Branch, Department of Transport.