The scope of this case-study was the development of driver's mental models of partial driving automation. For an adoption decision to be made, and to facilitate use of driving automation mental models are important. Systems such as Volvo´s Pilot Assist has been included as standard equipment in recent cars and drivers has the option to incorporate these systems in their driving. In our study drivers were interviewed when they had decided to buy a new car, at the delivery, one week after delivery, and finally after four months of owning it. The results suggest that mental models develop over time and may explain behaviour outcomes (system usage). Prior knowledge and the interplay between theoretical and practical experience are important factors for the development of the mental model and to help the driver over a threshold. A tentative conclusion is that drivers could be supported when introduced to automated driving systems.