What are the key factors?

Fuel efficiency
While the increase in car ownership has been offset to some degree by increase in fuel consumption efficiency of around 20% due to improvements in vehicle design, car usage behaviours, a counter-trend towards larger vehicles (four by fours, minivans etc), and increasing energy consumption from additional features such as air conditioning mean that average energy intensity per 100km has only fallen slightly in Europe and Japan, and by around 2% in the US (EU, 2002).