The classic divisions of environmental health by the vectors food, air and water lend themselves to studies of the causes of disease, but the best approach when looking for environmental health interventions is to focus on the organised areas of human activity that have the greatest impact on the environment — housing, employment, manufacturing and transport. Of all these, transport provides perhaps the greatest potential for health gain, at least in First World countries. The choices we make in transport bear directly on the health of the population.

During 2001, an average of 159 597 vehicles crossed the Sydney Harbour Bridge daily in both directions. This compares with an approximate average of 10 900 vehicles crossing the Bridge daily when it was opened in 1932 (Mr Barry Armstrong, Traffic Data Analyst, Traffic Information Group, Traffic and Transport Directorate, NSW Roads and Traffic Authority, August 2002, personal communication).

The most familiar risk to health in our car-dominant transport system is road-accident death and injury. In 1999, road accidents resulted in 509 deaths and 12 000 injuries in New Zealand,1 and, in Australia, 1759 deaths1 and an estimated 30 000 injuries requiring hospital treatment.2 About a third of the deaths occurred among children and adults under 25 years of age.1 In fact, these figures reflect recent declines in road accident fatalities in Australia and New Zealand, and one of the public health success stories of the second half of the 20th century. In both countries, fatality rates more than halved between 1970 and 2000. Road accidents are frequently attributed to speeding, carelessness and risk-taking, but changes in the behaviour of road users do not explain the drop in the road toll: more important factors have been better vehicle design (including seat belts), safer roads and fewer vulnerable road users (such as pedestrians, bicyclists and motorcyclists).3 By the standards of most OECD countries, however, our rates of road-accident deaths and injuries are still high. In 1999, road deaths per 100 000 were 13.3 (NZ) and 9.3 (Australia), compared with 9.7 (Canada), 6.6 (Sweden) and 6.0 (Britain).1 Furthermore, current patterns of industrialisation worldwide suggest that road accidents will become more prominent as a cause of death and injury. Already, more people are killed on the roads each year worldwide than die from malaria (2.3% v 1.9% of the global population).4 By 2020 road accidents are predicted to rank third among the causes of global disability-adjusted life-years lost, after cardiovascular disease and depression, and ahead of cancer.

Road accidents are nevertheless just the tip of the transport and health iceberg. The effects of vehicle emissions on public health have been considerably under-rated. New data on the relation between exposure to fine airborne particles and mortality suggest that the burden of disease attributable to traffic pollution may be at least as great as that caused by road accidents. In Europe, the number of premature deaths among adults caused by vehicle emissions was estimated to be more than twice the number of deaths from road accidents, albeit with rather fewer years of life lost.5 Using the same approach, in New Zealand we have estimated that about 400 deaths per year can be attributed to traffic pollution.6 By comparison, about 960 deaths per year in Sydney in 1989–1993 have been attributed to exposure to particulate pollution from all sources.7 Not counted in these analyses are the effects of global air pollution as a result of vehicles, roadbuilding and fuel production. The transport sector is a major contributor to climate change. Transport emissions — already responsible for 28% of total greenhouse emissions in New Zealand8 and 16% in Australia9 — are increasing more rapidly in both countries than those from any other sector (see also the article by McMichael and Woodruff [page 590] on climate change and health10).

Perhaps the most serious public health implication of our car-dependent societies is the unprecedented level of sedentariness that this lifestyle encourages. In cities, where the majority of Australians and New Zealanders now live, the proportion of trips made by walking, cycling or using public transport has plummeted. In the past 10 years, the proportion of New Zealanders cycling to school and work has fallen by more than 20%.11 In New Zealand and Australian cities, fewer than 5% of workers commute by bicycle, compared with 15%–20% in European cities.12 Fifty years ago, in New Zealand, there were 13 cars per 100 people; now, in both New Zealand and Australia, there are close to two cars for every three people. Some of the consequences of declining physical activity, such as increasing bodyweight and cardiovascular disease risk, are well known. Others, such as the dominating influence of trends in physical activity on the diabetes epidemic, are just becoming apparent.13 The evidence that lack of physical activity is strongly linked with certain cancers (colon and breast cancer, in particular) suggests that motor cars should be included in any up-to-date list of cancer-causing agents.14 A recent assessment of risk factors in Victoria suggested that lack of physical activity was outranked only by tobacco as a cause of ill-health.15

The Mount Victoria Tunnel connects the eastern suburbs of Wellington, NZ, with the city centre. When it was built in 1934, it was used each day by about 4000 vehicles and 2000 pedestrians. In 2001 there were about 34 000 (Health and safety aspects of the Mt Victoria Tunnel. 5th year medical student project. Wellington: Department of Public Health, Wellington School of Medicine and Health Sciences, 2001).

Current transport patterns in Australia and New Zealand are firmly entrenched. For almost every aspect of daily life — recreation, socialising, employment — we depend on using a private motor vehicle. As a result it is difficult to avoid trade-offs that make sense at the individual level, but in the broad scheme of things only make the problem worse. We have all heard self-justifying comments such as: "If everyone else goes shopping in a four-wheel-drive then I feel much safer in one too", or "I need to drive the kids to school because the roads are too busy for them to walk". How can we change this?

There are plenty of options that would result in benefit to the environment, the economy and human health. Examples include more fuel-efficient vehicles, and a better public transport system. Governments need to promote active transport strategies, such as supervised walking groups of schoolchildren ("walking school buses"), and workplace incentives to reduce car travel, such as public transport vouchers instead of company cars, and changing rooms and showers for cyclists. But major gains will require serious attention to the underlying structural factors that have produced our current high level of car-dependence12 and made walking and cycling more difficult and hazardous. These include removing economic subsidies that hide the true costs of roads and parking facilities, and limiting the urban sprawl that, to a large extent, dictates the use of cars. We need to think more imaginatively about how people can be helped to live close to the facilities they value. This means challenging many of the assumptions on which car-dependent city planning is based (such as the need for segregated land use and low-density housing). To achieve these goals there must be substantial redirection of transport spending from roads to other means of getting around, including public transport, footpaths and cycleways. Examples of positive changes that are now occurring include a less car-dominated National Transport Strategy in New Zealand, and initiatives such as New South Wales/Sydney Action for Transport 2010.

We recognise the extreme difficulty of making these changes — there are many powerful reasons why society is car-dependent, including perceived comfort, convenience and security. But the negative health aspects of our present transport system have been under-rated. As Peter Newman, Professor of City Policy at Murdoch University, has pointed out,12 the changes that have to be made require a critical change in mindset, from the privatised world of the motor car and the remote, car-dependent suburb, to the public realm of common spaces, community interests and a global environment that we all share, and on which our public health depends.