Around the world, road traffic is the dominant anthropogenic source of air pollution in urban areas. It is growing concern in many countries due to increased car ownership, growth in traffic in urban areas, and increased congestion levels. This trend is evident despite stricter vehicle emissions standards and associated reductions in vehicle emissions and tightening of air quality standards. 

In Australia, about 14% of total greenhouse gas emissions come from transport. The majority (80%) of these emissions come from road transport and the remainder from aviation, shipping and rail. Road traffic is particularly important since it emits large quantities of harmful chemicals close to the population. 

Moreover, total emissions from transport are growing strongly. Transport CO2 emissions have increased by 30% in the period 1990-2005 and they are expected to almost double in the period 1990-2020. The fastest growth is expected for aviation. 

The increase in transport pollution poses significant challenges for policymakers, and makes policy development and impact assessment of new infrastructure increasingly important. The scale of interest varies from local road projects to entire (urban) transport networks and even national or global emission inventories.