Abstract The planning of any major traffic scheme should be preceded by economic calculations of the benefit to be derived from it. In this connection, a distinction can be made between the direct benefit accruing to road users from the new facilities compared with existing facilities, and the indirect effect which the facilities will have on the areas served by them. The benefit accruing to road users is measured by the savings resulting from the new facilities in terms of time, fuel consumption, tyre wear, repairs etc. and by the greater travelling comfort and reduced accident risk. To the extent to which such benefits can be economically quantified, an economic balance sheet can be drawn up and, by comparing the benefits with the costs, a “cost/benefit ratio” can be worked out so as to decide whether the scheme is economically justifiable. More difficult is the economic assessment of the indirect effect of a traffic scheme. In recent years, however, extensive research work has been carried out in the United States to try and assess the economic and social impact of highways on the areas served by them. In this article, the attempt is made to establish and test, by means of a “macro-model”, a working theory for assessing the impact of improved traffic facilities on the welfare of the areas affected by the improvement. The theory is then used for assessing the indirect effect of two alternative schemes for the construction of motorways in Denmark.