For power transmission systems compliant with safety codes and reliability standards, there remains a risk of tree-caused interruptions from the in-fall of trees from outside the right-of-way. This paper reports on the quantification of tree exposure outside National Grid’s transmission corridors and examines the variables impacting the risk of a line contact by trees. Correlations between the variables and National Grid’s tree-caused interruption experience were tested. Regression analysis was applied to a calculated risk factor and the annual interruption frequency. Two mitigation approaches are compared for cost and efficacy in improving line security. One is based on a regulator suggested use of minimum right-of-way width, while the other is site specific, based on specific site risk versus the voltage class mean risk.