The growth of traffic in cities is leading to rising levels of congestion. The provision of new infrastructure is often not a solution, since it can generate more traffic and make the urban environment less attractive. Another approach is to make better use of the existing infrastructure, by filling the unused space in vehicles. For example, in peak period commuter traffic, average car occupancy rates may be as low as 1.2. Therefore car-pooling seems attractive, where at least two people ride in a private car usually owned by one of the occupants. (This is also known as car sharing in the UK.) However, little is known about people's attitudes towards car-pooling and how best to promote this behaviour.