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.