This paper presents a new product design method that gives users a proactive role in the design process. Within a dedicated design environment, users are allowed to create their own designs and immediately test these in a wide variety of scenarios. By letting users realistically interact with their personal creations, designers can quickly and reliably pinpoint their needs and preferences. At the same time, good designs are generated. To evaluate the new method, it was applied to a design case: the design of a lane change support system. It was found that the new method offers added value for the design of driver support systems.