In 2004, transport accounted for almost a quarter of carbon dioxide (CO2) emissions from global energy use.1 Three-quarters of transport-related emissions are from road traffic.1 Although large reductions in greenhouse-gas emissions are needed to prevent serious climate destabilisation,2 emissions from transport are rising faster than from other energy-using sectors and are predicted to increase by 80% between 2007 and 2030.1
Reduction in transport-related greenhouse-gas emissions through less use of motor vehicles and increase in the distances walked and cycled could have important health benefits.3 Reduction in the use of motor vehicles could reduce urban air pollution. Prevalence of physical inactivity and the associated burden of chronic disease could be lowered with increases in the distances walked and cycled.4 Decrease in motor vehicle traffic also has the potential to reduce danger from road traffic, although exposure to the remaining danger might increase with the number of pedestrians and cyclists.5 However, the extent of these effects is not known.
We modelled the effects of urban land transportation scenarios on CO2 emissions and health. Motor vehicles are a source of several other climate-active pollutants, including black carbon, ozone (indirectly), nitrous oxide, and methane. In this Series, Smith and colleagues6 discuss the climate and health implications of several of these pollutants. However, we have restricted our analysis to CO2, and modelled emissions only from motor vehicle fuel combustion; full life-cycle modelling was beyond the scope of this analysis.
We focused on urban transport because more than half the world's population lives in cities and because we expected the potential for change and health effects to be greatest in cities. In low-income and middle-income countries, urbanisation is associated with an increased health burden from non-communicable diseases.7 In the UK, transport in urban areas accounts for 20% of distance (km) travelled by vehicles,8 but accounts for a disproportionate share of CO2 emissions and air pollutants as a result of the driving conditions9 and frequent vehicle cold starts.10
Key messages
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Transport-related greenhouse-gas emissions are increasing, with a rapid growth projection in low-income and middle-income countries.
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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.
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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.
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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 ischaemic heart disease, cerebrovascular disease, depression, dementia, and diabetes.
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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.
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Creation of safe urban environments for mass active travel will require prioritisation 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.
We assessed physical activity, outdoor air pollution, and risk of road traffic injury. Although transport can affect health in other ways, including noise pollution, community severance, and the opportunity cost of transportation resource use,3 the three exposures were selected because the evidence linking them with health outcomes is strong. Figure 1 shows the pathways that were included and excluded.