Purpose of the research: Environmental aspects have become increasingly important in air traffic. In the ANSP context, horizontal flight efficiency has been used in a regulatory framework. Recently, a new metric called "excess fuel burn" (XFB) was introduced for analysis. We will show to what extent both metrics are suitable for assessing environmental performance. Research design, methodology, or approach: The paper focuses on how the two metrics are calculated and which components are included. Regression analysis will be used to determine which factors have a significant impact on the environmental performance indicators, distinguishing between endogenous and exogenous effects. The results allow for an evaluation of the indicators regarding meaningfulness and validity. Research findings: Qualitative as well as quantitative analysis shows which factors influence the metrics, and how. As a result, both metrics have strengths and weaknesses. Nevertheless, there is some exogenous effect on the indicators. We will derive recommendations for suitable indicators to measure ANSP-induced inefficiency. Contribution of the research paper: Environmental factors will play an important role in both benchmarking activities and target setting within the regulation periods. However, to increase efficiency, valid measurement methods are necessary to validly determine influencing factors and to derive effective measures to improve efficiency. We, therefore, propose a new metric (3DE) to address environmental performance.