The research systematically compared the driving performance and conversational patterns of drivers speaking with in-car passengers, handsfree cellphones, and remote passengers who could see the driver’s current driving situation (via a window into a driving simulator). Driving performance suffered during cellphone and remote passenger conversations as compared with in-car passenger conversations and no-conversation controls in terms of their approach speeds, reaction times, and avoidance of road and traffic hazards. Of particular interest was the phenomenon of conversation suppression, the tendency for passengers to slow their rates of conversation as the driver approached a hazard. On some occasions these passengers also offered alerting comments, warning the driver of an approaching hazard. Neither conversation suppression nor alerting comments were present during cellphone conversations. Remote passengers offered some alerting comments but did not display conversation suppression. The data suggested that conversation suppression is a key factor in maintaining driving performance and that visual access to the driver’s situation is not sufficient to produce conversation suppression. A second experiment investigated whether a cellphone modified to emit warning tones could alleviate some of the adverse effects typically associated with cellphone conversations. The modified cellphone produced discourse patterns that were similar to passenger conversations and driving performance nearly as good as that of drivers who were not conversing.