The rapid development of driving automation systems (DAS) in the automotive industry aims to support drivers by automating longitudinal and lateral vehicle control. As vehicle complexity increases, it is crucial that drivers comprehend their responsibilities and the limitations of these systems. This work investigates the role of the driver's perception for the understanding of DAS by cross-analysing four empirical studies. Study I investigated DAS usage across different driving contexts via an online survey conducted in Germany, Spain, China, and the United States. Study II explored contextual DAS usage and the factors influencing drivers' understanding through a Naturalistic Driving Study (NDS), followed by in-depth interviews. Study III employed a Wizard-of-Oz on-road driving study to simulate a vehicle offering Level 2 and Level 4 DAS, paired with pre- and post-driving interviews. Study IV following up used a Wizard-of-Oz on-road driving study to simulate Level 2 and Level 3 DAS and subsequent in-depth interviews. The findings from these studies allowed the identification of aspects constituting a driver's understanding and factors influencing their perception of DAS. The identified aspects and factors were consolidated into a unified conceptual model, describing the process of how perception shapes the driver's mental model of a driving automation system.