When developing increasingly complex assistance systems, the functional safety of these systems plays an important role. To minimize the risks of malfunctions, safety-related systems should be developed, tested and verified in line with relevant standards (such as ISO 26262). A key component of this process is evaluating the controllability of the systems. We offer custom investigative concepts for evaluating the controllability of systems and assessing the criticality of system failures, both in real vehicle and driving simulator environments. This enables us to provide early support to the development of safe systems. Benefit from our many years' experience conducting clinical studies on topics such as faulty braking interventions and faulty auxiliary steering moments. Our methods put the driver in the focus of our assessment process, allowing us to identify potential problem areas from a controllability aspect and propose targeted solutions.