Elsevier

The Lancet

Volume 374, Issue 9705, 5–11 December 2009, Pages 1930-1943
The Lancet

Series
Public health benefits of strategies to reduce greenhouse-gas emissions: urban land transport

https://doi.org/10.1016/S0140-6736(09)61714-1Get rights and content

Summary

We used Comparative Risk Assessment methods to estimate the health effects of alternative urban land transport scenarios for two settings—London, UK, and Delhi, India. For each setting, we 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. We developed separate models that linked transport scenarios with physical activity, air pollution, and risk of road traffic injury. In both cities, we 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 (7332 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 1696 in Delhi). However, combination of active travel and lower-emission motor vehicles would give the largest benefits (7439 DALYs in London, 12 995 in Delhi), notably from a reduction in the number of years of life lost from ischaemic heart disease (10–19% in London, 11–25% in Delhi). Although uncertainties remain, climate change mitigation in transport should benefit public health substantially. Policies to increase the acceptability, appeal, and safety of active urban travel, and discourage travel in private motor vehicles would provide larger health benefits than would policies that focus solely on lower-emission motor vehicles.

Introduction

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

  • 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 ischaemic 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 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.

Section snippets

Modelling the scenarios

We designed scenarios with reference to a large city in a highly motorised country (London, UK), and a large city in a country that is becoming rapidly motorised (Delhi, India).

For London, we developed four scenarios and compared them with a business-as-usual 2030 projection (panel 1; webappendix p 9). In the lower-carbon-emission motor vehicles scenario, we focused on reducing the emission factors from motor vehicles. The increased active travel scenario represented a large increase in

Modelling health effects

For all scenarios, we estimated the distributions of physical activity and exposure to air pollution. We then used the methods of Comparative Risk Assessment (webappendix pp 6–8) to estimate the change in disease burden. A modified approach was used for road traffic injury in which we calculated absolute numbers of deaths. Although we started with projected data for disease burden for 2010, we compared each mitigation scenario against the 2030 business-as-usual transport scenario—ie, if 1000

Findings

Evidence from systematic reviews showed that increased physical activity reduced the risk of cardiovascular disease, depression, dementia, diabetes, breast cancer, and colon cancer. Table 5 shows the results of our overview, strength of the association between the exposure and outcome, and estimates used in the modelling (webappendix pp 27–31).

All these conditions, except for depression and dementia, were included in the earlier Comparative Risk Assessment study of physical activity (search

Strengths and weaknesses

We noted that a scenario that represented a move towards sustainable transport could provide substantial reductions in chronic diseases, including ischaemic heart disease, stroke, depression, and dementia. The health gains were larger from increases in active travel and reductions in use of motor vehicles than from use of lower-carbon-emission motor vehicles.

Panel 3 shows the key assumptions used to model the scenarios. Our estimates of health effects depend crucially on the structure and

Implications for policy

Effective policies to increase the distances walked and cycled and reduce use of motor vehicles are needed to achieve the health benefits we have discussed. Policies that encourage people to walk and cycle would be expected to increase the safety of active travel, as shown in our sensitivity analysis of injury risks in the Netherlands.22 Substantial increases in the distances cycled in cities, including Copenhagen (Denmark), London, and New York (USA), are associated with a decrease in the

Conclusions

Important health gains and reductions in CO2 emissions can be achieved through replacement of urban trips in private motor vehicles with active travel in high-income and middle-income countries. Technological measures to reduce vehicle pollutants might reduce emissions, but the health effect would be smaller. The combination of reduced reliance on motorised travel and substantial increases in active travel with vigorous implementation of low-emission technology offers the best outcomes in terms

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