The threat of increasingly unsustainable transportation systems in the large cities of
the world comes from the interaction between growing demand for transportation
services and the environmental impact of transportation. Demand tends to grow at a
geometric rate, while the environmental capacity (at given technologies) to handle
such growth is fixed. Technology offers enormous possibilities for change in the
longer run, but unless harnessed to the goal of sustainability, may aggravate some
problems (such as traffic congestion) while in the process of fixing others (reducing
emissions per road vehicle kilometre).