An in-vehicle speech dialog system (SDS) can support visualhaptic interfaces and reduce eyes-off-road time while driving. This work evaluates two SDS that varied according to the degree of natural language understanding afforded by the speech dialog system. In a Wizard of Oz simulation, two alternative SDSs were tested in a driving simulator. The Lane Change Test was used to compare a command and control system with a system supporting natural language input. This driving simulator study was conducted using the same setup in Germany, USA, and China. 40 participants per country were instructed to perform interaction tasks from contexts like media, telephone, and navigation. The results show that natural language SDS could lead to a faster and more intuitive way of interacting with in-vehicle SDS. US and Chinese users especially preferred the natural language enabled system over the command and control system.

Though legislation is increasingly discouraging drivers from holding on to their mobile phones while talking, hands-free devices do not improve driver safety. We offer two design alternatives to improve driver safety in the contexts of voice-based user interfaces and mobile phone conversations in cars' side tones (auditory feedback used in landline phones) and location of speakers. In a 2 (side tone: present vs. not) x 2 (location of speakers: headphones vs. dashboard) between-participants experiment (N=48), we investigated the impact of these features upon driver experience and performance on a simulated mobile phone conversation while driving. Participants became more verbally engaged in the conversation when side tones were present, but also experienced more cognitive load. Participants drove more safely when voices were projected from the dashboard rather than from headphones. Implications for driver user interface design are discussed.