Design is about practically imagining what could be rather than what already is. This is problematic for designers in the transport industry, because they can find themselves working to conflicting and shifting design specifications. However, at the same time as concerns about the environmental impacts of transport grew in importance, a more entrenched and economically powerful factor played a key role in changing the nature of transport planning. In China, the most common cause of death is now respiratory illness brought on by air pollution, of which transport is a major contributor. Transport planning had to change to understand and manage the system generating travel demands. The shift towards demand management transport planning was reinforced by the growing awareness through the 1990s of transport's emissions of carbon dioxide (CO2) and other 'greenhouse gasses'. One reason for developing a real strategic approach to transport planning was that, the environmental impacts of transport become increasingly apparent.