This paper describes an Air Traffic Control (ATC) cognitive complexity indicator yielded from a human-in-the-loop simulation. The analysis used to this end employs factor analysis for the identification of the correlation between cognitive complexity and several cognitive complexity and workload metrics. Thereby, a single complexity indicator has been developed. It has proved to be more responsive to the overall complexity than any of the other subjective metrics alone. Nonetheless, there is indication that the proposed complexity indicator must not be taken alone, but analyzed together with other metrics. The paper also tackles the impacts of traffic history, automation and optimized schedules on the perceived complexity and how these impacts should be taken into consideration on the design of future concepts of ATC operations. This is done through the application of the proposed indicator into several ATC scenarios.@en