Sustainable Transport
Related terms: emissions, equity, impacts, infrastructure, mobility, politics, public engagement, resilience, sustainability
The sustainable transport challenge
The term ‘sustainable development’ refers to “economic development in which natural resources are used in ways compatible with the long-term maintenance of these resources, and with the conservation of the environment” (OED 2024). For instance, adopting recycling programmes to reduce waste or installing wind turbines to generate energy. Unfortunately, under such a definition, transport is currently categorically not sustainable.
Thus, in 2022, the transport sector used 117 exajoules of energy worldwide, of which ~90% was from oil. This accounted for 28% of the total energy consumed, which was second only to the industrial sector (IEA 2024), whilst in 2023, transport produced 21% of all carbon emissions from combustion (Crippa et al. 2024). In addition, 7 million premature deaths worldwide occur annually through exposure to air pollution (including from transport) (WHO 2021); 1.2 million people are killed and between 20 and 50 million more are injured each year by road traffic collisions (WHO 2023); and 0.5% to 8.5% of worldwide GDP annually (i.e., $US530 billion to $US9.01 trillion, 2023 values) is lost annually due to delays arising from the effects of traffic congestion (Statistics Times 2024; Schreyer et al 2004; World Bank 2025). Meanwhile the current hyper-mobile society paradigm we live in also results in reduced social cohesion due to dispersed development, blight and community severance effects; reduced social opportunities for those without access to cars due to reduced use (and poorer quality) of public transport, walking and cycling experiences; and increased stress levels, mental health issues, pulmonary health complaints, and obesity rates in adults and children (Enoch 2012; Gunn 2018). Also important is the need to preserve the health of the planet through maintaining biodiversity (Mladen et al. 2024). Problematically, urban expansion, roads, railways, airports and seaports fragment and destroy habitats and restrict species migration and reproduction, whilst vehicles contribute to pollution and the spread of invasive species, further destabilising ecosystems. Efforts therefore need to be made to minimise the environmental impact of transport through lowering emissions, reducing resource consumption, developing eco-friendly infrastructure, limiting urban sprawl, and ensuring the transport system is accessible and equitable.
Frustratingly however, and unlike almost every other sector where the figures are levelling off or even falling in many parts of the world, trends in energy and carbon emissions for transport are moving in the wrong direction despite the internationally recognised urgency in reducing them to stave off the worst effects of global warming (IPPC 2023). Thus, car ownership is set to increase from 1.3 billion cars in 2020 to 2.8 billion in 2050 (average annual growth rate of 3.8%); air travel from 3.31 billion passenger seat miles to 15.1 billion (11.9% p.a.); energy use from 108 quadrillion British Thermal Units to 155 quadrillion (1.5% p.a.); and carbon dioxide emissions from 7.15 giga tonnes of CO2 equivalent to 9.23 giga tonnes (1.0% p.a.) over the same 30-year period (USEIA 2021; Enoch 2025).
So why is sustainable transport so challenging to deliver?
Characterising transport
To answer this, it is helpful to look at the characteristics of transport.
The function of transport from an economist’s perspective is “to move passengers or goods from where they are to where they would prefer to be or to where their relative value is greater… [This] demand for transport arises from the uneven distribution of raw materials, skills and labour” (Faulks 1973, 16). Viewed through a ‘more-than-human’ political ecology lens, recognition is afforded to the agency and value of non-human actors (air, water, animals, and landscapes) as an ecological process (Joslin 2024). Ecological transport, e.g. the flow of air or rivers, the migration patterns of animals, or the processes of erosion and deposition, is directed at maintaining the metabolic balance of ecosystems. Meanwhile, in Marxist ecological thought, capitalist modes of production and consumption result in a disruption of natural cycles – the idea of a ‘metabolic rift’ (Moore 2025). Transport contributes to this through the extraction of mineral resources for fuel and the destruction and fragmentation of habitats by infrastructure.
Alternatively, “transport can be said to involve movement of people and goods, be purposeful, and (generally) only occur as a consequence of other activities taking place in different locations at different times” (Enoch 2012, 1). This definition implies that transport involves matching the demand for transport with the supply of transport (Ortuzar and Willumsen 2001).
Transport demand is a function of the ‘need’ for transport within a particular community; hence, more people will likely need more transport. Travel is also often said to be derived from other activities or purposes. It is not usually an end in itself. Hence, transport planners must understand how work, education, leisure, and retail activities are distributed over time and space (Cole 2006). Worldwide increases in population, urbanisation, and economic activity underpin the high predicted growth rates of transport, energy and carbon emissions up to 2050.
Transport demand has a temporal component, i.e., it is dynamic, generally varying over a day, throughout the week, from season to season, and throughout the year, depending on the activities being undertaken. This uneven distribution of demand is challenging for transport providers, who are likely unable (or unwilling) to operate sufficient services during peak times (leading to overcrowded vehicles and/or congested rights of way) or during quieter periods (meaning reduced travel opportunities for users) (Vuchic 2007).
Similarly, the spatial distribution of activities heavily influences transport demand, and it can lead to problems where, for example, there may be too many taxis and not enough customers in one area, and too many customers and not enough taxis in another. One approach to addressing this is to divide study areas into zones and then relate them to transport networks (Hanson 1995).
Adding to this complexity, the demand for transport is differentiated by the circumstances and preferences of each user at the moment they choose to make a journey, and by the fact that it is qualitative, whereby lots of attributes are not quantifiable. In other words, many people (and politicians) make travel (and investment) decisions that are not based solely on strictly rational criteria. Instead, transport choices for many people are strongly rooted in socio-cultural contexts and are influenced by identity, image, glamour, sex appeal, class, race, gender differentiation and more (Root 2003). Unfortunately, it is often the least sustainable transport choices (e.g. driving solo in a heavy, large-engined Sports Utility Vehicle, long-distance air travel) that best represent the most sought-after attributes: connection, the freedom to explore, independence, development, growth, new experiences, and communication. By contrast, one particularly powerful image depicts the much more sustainable bus as being a “loser cruiser” (Fitt 2018, 228).
On the supply side, transport systems comprise ‘mobile units’ or ‘vehicles’ (e.g. cars, trucks, buses, bicycles, trains, aircraft) that are propelled, guided and controlled along set pathways or ‘rights of way’ by entities collectively known as ‘fixed elements’ or ‘infrastructure’, according to agreed rules (Vuchic 2007).
Transport is a service and not a good. In other words, transport cannot be stored but is perishable and must be consumed when produced. It cannot be ‘saved up’ for times of peak demand. Hence, it must be predicted as accurately as possible to avoid that resources will be wasted (Ortuzar and Willumsen 2001).
As with demand, transport supply is heavily dependent on the external environment. The evolution of transport has been especially driven by technology. On the passenger side, this has progressed from walking through animal traction, canals, railways, trams, motorised buses, and cars to aircraft. This increasing specialisation and separation of ‘transport modes’ has progressively raised travel speeds, much of which has then translated into longer journeys and more distributed land use patterns, rather than into shorter journey times (Garrison and Levinson 2014). Similar developments have taken place relating to freight.
Partly as a consequence, transport systems are often highly capital intensive, requiring not only significant financial resources, but also labour, land, materials, equipment, time, carbon, energy, and indeed political capital to plan, build, operate, maintain and upgrade/replace (Button 2010). Heathrow Airport is one such example of a legacy penalty: expansion of what is a sub-optimal location for a large international hub airport is very challenging, but relocation to a more appropriate site has proven even less appealing. Such resigned acceptance that nothing can change tends to reinforce or ‘lock in’ existing technological paradigms, even if they are no longer necessarily the best solution (Geels 2005).
Closely related to this, the provision of transport supply is lumpy: one cannot provide half a runway or a third of a bus. It is often, therefore, difficult to match the supply with the demand (Ortuzar and Willumsen 2001).
Politically, the strategic objectives driving transport policy include stimulating economic growth and productivity, improving social cohesion and public health and protecting the environment by minimising: energy use, carbon emissions, local air pollutants, noise, community severance, land take, and damage to natural and heritage sites (Enoch 2012). Tactically, these can be achieved through planning communities to minimise the need for people to own and use a car, improving journey time reliability by reducing congestion, enhancing accessibility and connectivity by increasing network coverage and levels of service, proactively managing traffic to reduce traffic crashes, and enabling people to walk and cycle more often.
Delivering these goals is not helped by the fact that the supply of transport is typically provided by public, private and sometimes community-based organisations. These include multiple national and local-level government departments and agencies, as well as operators, developers, businesses, and travellers, each with their own (often conflicting) goals (Headicar 2009).
One long-standing issue is that travellers often do not pay the full economic cost of using the transport system, meaning it is not used as efficiently as it might be. This fact helps explain the continued growth in transport use, energy and carbon emissions (Button 2010).
Finally, for analysis purposes, transport systems are often conceptualised as networks, where nodes equate to junctions and terminals, and links to the paths between them. Flows of pedestrians, cyclists or vehicles can then be assigned either as aggregate hourly or daily values, or increasingly as independent agents (Hanson 1995).
Making transport sustainable
Making the transport system sustainable will require nothing short of a paradigm shift, both amongst the consumers and the providers of transport. This must comprise the following elements: perspectives, ideas, and narratives.
Traditionally, transport has been largely studied by engineers, economists, geographers, computer scientists, operational researchers, planners and mathematicians as a system or a process used (and operated) by rational actors. Only relatively recently have we seen work by behavioural scientists and human-centric designers pay attention to how and why people behave as they do in particular circumstances. Arguably, there is a need for broadening these perspectives much further, and incorporating more creative viewpoints – from history, design, sociology, politics, languages, art or the environmental humanities.
Next, we need to better understand not just what will likely happen in future (according to machine learning-based predictions), nor what should perhaps happen in future, important though these tasks are. Rather, we should also consider what transport could look like – 10, 30, 50 years ahead. Only in this way can we begin to think about changing course away from the current path we are blindly following.
Finally, promising, strongly sustainable scenarios must be embedded into narratives that are sufficiently convincing to change the minds of decision-makers and other key stakeholders, and persuade them to make the changes necessary to replace our current highly destructive habits.
In terms of good practice sustainable transport exemplars, Copenhagen in Denmark is a global leader in cycling, congestion reduction policies and electrified public transport systems. The urban structure of Curitiba in Brazil, meanwhile, was largely built around a high-capacity busway network, with high-density developments located alongside these public transport corridors, making bus, walk, and cycling journeys more convenient than car journeys. Paris, the French capital, has prioritised pedestrianisation schemes, investment in cycling infrastructure and a shared bicycling scheme, improvements to the light rail and metro systems, and restrictions on car use to improve accessibility and reduce the impacts of transport on the environment.
References
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