KIA has done a survey (with over 2000 people) in the UK on decision making in the car. It appears that people use the time in the car to discuss major issues in their lives and that they make significant decisions during long journeys. I have not found the original survey from KIA but there are several pages that discuss the results, e.g. gizmag.
Some findings in short, people talked about/made descions: going on holiday (63%), buying a car (50%), moving (40%), getting a pet (26%), getting married (23%). The main reason for the car on a long journey being an effective environment for communication seems the fact the people are close together for a long time and no-one can walk away (41%). Also the fact that you have reason not to look the other person into the eyes, as you have to watch the street, was valued.
Thinking about it there it may also have to do with the function of space. A car puts people close together – in some case to intimate distances (up to 50cm) but defiantly to personal distances (50cm-125cm). There is a comprehensive overview by Nicolas Nova, Socio-cognitive functions of space in collaborative settings: a literature review about Space, Cognition and Collaboration (original reference to my knowledge is Hall, E.T. (1966). The Hidden Dimension: Man’s Use of Space in Public and Private. Garden City, N.Y.: Doubleday.).
This survey made me think more about the design space "car". Recently two of my students - Anneke Winter and Wolfgang Spießl - finished there master projects at BMW looking into search technologies and user interfaces in the car. It seems there are a lot of ideas that can be pushed forward realizing Ubicomp in the car.