Proposition a model for estimating the environmental impacts (externalities) Department of transportation in Tehran

Positive and negative externalities are one of the most important issues in economic analysis. In economics، an externality، or transaction spillover، is a cost or benefit not transmitted through prices that is incurred by a party who did not agree to the action causing the cost or benefit. In this article we try to estimate the effect of extension of urban transportation on Carbon monoxide in the air of Tehran as a positive of negative externality. The models in this article are estimated by ordinary least squares method. According to the results in both estimated models positive externality and Gross domestic product has negative externality.