Designing computer-human interfaces for multiple-goal environments is challenging because people pursue multiple goals with conflicting priorities. Safety-critical environments, such as driving, aggravate the need for a more nuanced understanding of interfaces that may reconcile conflicting tasks. Speech interfaces are prime examples of such interfaces. In this article, we investigate how design variations of an in-vehicle speech interface influence performance of a primary task (driving safely) and a secondary task (e-mailing). In a controlled experiment, we test the performance implications of using single computer-generated Text-To-Speech (TTS) voice and multiple matching TTS voices while users respond to e-mails with varying levels of complexity during driving. Our results indicate that the number of voices used has a significant effect on both driving performance and handling e-mail--related activities. We discuss potentially unintended consequences of making the interface too naturalistic and too engaging for the driver and conclude with theoretical and practical implications.

Advancements in information and communication technology make new IT-based services in the tertiary scope of a car driver (short: automotive services) possible like e-mail and social networking and are highly demanded by customers. However, if not well designed, these services have a potential to dangerously distract drivers from their main driving task and therefore increase the risk of road accidents. Hence, we studied the effects of an in-car e-mail client on drivers' distraction. A low fidelity driving simulator was used to test 32 participants on their degree of distraction while driving. In this paper we present the findings of this evaluation. Our results show that when designing complex automotive services like an e-mail client one of the crucial factors besides the user interface is to keep the provided content simple and well structured thus reducing the amount of driver's distraction considerably.