It is common understanding that human-computer interaction (HCI) systems should be designed unified. However, ensuring a unified interaction design (UID) is a cost intensive and time-consuming venture. Especially the automotive industry struggles with exceeding costs and time-to-market pressure as drivers want to stay connected and informed while driving. Therefore, we investigated the effect of non-unified interaction design (NUID). We report on a simulator study with 44 participants in which we studied the effect of a NUID within an automotive HCI system consisting of five in-car applications. We measured the effect on driving performance, task performance and situational awareness when carrying out tasks. We found no significant effect of UIDs. We offer an explanation based on HCI and cognitive ergonomics literature.