It has been demonstrated that cell-phone conversations impair driving performance. However, it is unclear whether the difficulty of naturalistic phone conversations can modulate driving performance. The present study used a simulator to investigate whether the cognitive load of phone conversations (non-conversation, multiple choice and open question conversations) influence highway driving performance. The results showed cell phone conversations with open questions led to most aggressive driving with highest speeds and shallowest braking. Furthermore, open question conversations led to the smallest route deviations. These results suggested that a drivers' capability for monitoring speed and making manoeuvre decreases as the difficulty of a phone conversation increases. The implications of this study for driving safety are discussed.