<p>The air transportation system is a constantly evolving complex socio-economic entity, whose performance requires detailed monitoring. The well-established performance-based framework by the International Civil Aviation Organization (ICAO) pushed the development of various indicators to assess the Air Traffic Management (ATM) performance. This paper aims to propose a set of specific indexes to monitor the impact of Call Sign Similarity (CSS), an area under-investigated in literature. A CSS is recognized as a safety hazard that verifies when two flights share similar Call Signs, with the potential to generate Call Sign Confusion events, i.e., cases where instructions are mis-guided for two aircraft with CSS. For this purpose, two Key Performance Indicators (KPIs) are proposed to evaluate airspace performance with implications for capacity and safety by means of: frequency of CSSs, and CSS overlapping time in the same sector. Early results obtained from the implementation of these two KPIs are provided in a use case for the European airspace, demonstrating their usability for ATM strategic planning accounting for both operational safety and airspace complexity.</p>