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.