What are the key factors?

Intercity travel
Analysing intercity travel is complicated by definitions of what constitutes an intercity trip, which varies between countries, and differentiating local and intercity trips by mode of transport. It is clear however that intercity travel has been increasing both in terms of absolute distance and as proportion to total passenger kilometres, with US figures indicating a rise from 17% of distance travelled in 1977 to 19.5% of in 1995 (WBSCD, 2001). In the developing world, bus travel is still an important means of long distance travel but has declined considerably in the developing world (WBSCD, 2001). The demand for personal convenience and to travel greater distances more quickly has lead to a rapid increase in the last 30 years in air travel and the construction of high-speed rail links. The number of intercity trips taken by cars has also increased by around 25% although the average distance for a car journey has remained around 850 km (WBSCD, 2001).