The paper proposes an approach for evaluating the effect of flow fixed costs on the evaluation of environmental benefits with travel cost method. On a full annual perspective when recreational users incur relevant annual direct fixed expenses, their behaviour could be influenced by them. The approach introduces a) the notion of the minimal number of annual visits that justifies the annual fixed expenses incurred by the user and b) a method to estimate it. The estimate of this minimal number permits to forecast the user behaviour on a full annual perspective, taking into account a more accurate estimate of the number of visits at different additional fees.