DfT research suggest that driving with a passenger is a benefit rather than distraction
The research sent out 5 men and 5 women on two 30 minute journeys during rush hour traffic, one route with a passenger and the other without. Various sensors were attached to the subjects to measure their stress levels. The results show that those with a passenger had lower stress levels at the start of the journey and managed to relax quicker at the end of the drive. Not only is there an environmental benefit to car-sharing but an emotional benefit as well.