For each setting, the authors of the study compared a business-as-usual 2030 projection (without policies for reduction of greenhouse gases) with alternative scenarios: lower-carbon-emission motor vehicles, increased active travel, and a combination of the two.

They developed separate models that linked transport scenarios with physical activity, air pollution, and risk of road traffic injury. In both cities, they noted that reduction in carbon dioxide emissions through an increase in active travel and less use of motor vehicles had larger health benefits per million population (7,332 disability-adjusted life-years [DALYs] in London, and 12,516 in Delhi in 1 year) than from the increased use of lower-emission motor vehicles (160 DALYs in London, and 1,696 in Delhi).

However, the combination of active travel and lower-emission motor vehicles would give the largest benefits (7,439 DALYs in London, 12,995 in Delhi), notably from a reduction in the number of years of life lost from ischemic heart disease (10—19% in London, 11—25% in Delhi).

Key messages from the study include:

• Transport-related greenhouse-gas emissions are increasing, with a rapid growth projection in low-income and middle-income countries.

• Production of lower-emission motor vehicles (cars, motorcycles, and trucks) and reduction in travel by motor vehicles are needed to meet targets for reduction of greenhouse-gas emissions.

• Lower-emission motor vehicles would reduce the health burden from urban outdoor air pollution, but a reduction in the distance travelled by motor vehicles could have a greater effect.

• Increase in the distances walked and cycled would also lead to large health benefits. Largest health gains would be from reductions in the prevalence of ischemic heart disease, cerebrovascular disease, depression, dementia, and diabetes.

• Although reducing motor vehicle use would decrease the injury risk for existing pedestrians and cyclists, if many more people walked and cycled there might be an increase in the number of pedestrian and cycle injuries, since more people would be exposed to the remaining risk.

• Creation of safe urban environments for mass active travel will require prioritization of the needs of pedestrians and cyclists over those of motorists. Walking or cycling should become the most direct, convenient, and pleasant option for most urban trips.

With our assumptions about model structure and the uncertainties in the model variables, the results of this study should be regarded as provisional and should be revised when more accurate estimates become available...The extent to which our results can be generalized to other cities is open to question. For example, London and Delhi are megacities with high levels of public transport use, which suggests that they are likely to have more walking and lower carbon emissions per person than cities with lower levels of public transport use. In cities with higher car use, the emission cuts needed would be increased but the health benefits could be even greater.

...Because we have estimated the health effects of scenarios rather than specific interventions we cannot assess cost effectiveness. However, the infrastructure for individuals to walk or cycle might be less resource-intensive than that for cars. Additionally there are likely to be direct and indirect economic and social effects that cannot be adequately addressed here. A key consideration is whether such cities could, with low resource use, achieve social goals.

—Woodcock et al.

Resources

• James Woodcock et al. (2009) Public health benefits of strategies to reduce greenhouse-gas emissions: urban land transport. The Lancet, Early Online Publication, doi: 10.1016/S0140-6736(09)61714-1