This study examines whether characteristics of a car voice can influence driver behavior and attitudes, and furthermore, if the driver's emotional state will influence the perception of the car voice. Participants in a 2 (driver emotion: happy or upset) x 2 (car voice emotion: energetic or subdued) experimental study, had emotion induced by video clips and then spent 20 minutes using a driving simulator. While they were driving, a voice in the car spoke 36 questions (e.g., "Have we passed the sign for Lucia yet?") and comments ("My favorite part of this drive is the lighthouse.") in either an energetic or subdued voice. Participants were invited to interact with the car voice. Matching the car voice to the drivers' emotions had enormous consequences. Drivers who interacted with voices that matched their own emotional state had less than half as many accidents on average as drivers who interacted with mismatched voices! Drivers paired with matched voices also communicated much more with the voice (the voice sid exactly the same thing in all conditions). The effects of matching emotion versus mismatching emotion were so powerful that neither driver emotion nor gender had an expected effect on driving performance. There was a slight tendency for happy drivers and female drivers to be better drivers, even though this effect was minimal compared to the effects of matching. In other words, finding the appropriate in-car voice for the driver's emotion stood out as the most critical factor in enabling a safe driving experience.