Author: Dr. Jean-Paul Rodrigue
Sustainable transportation is the capacity to support the mobility needs of a society in a manner that is the least damageable to the environment and does not impair the mobility needs of future generations.
1. Sustainable Development
a. The concept of sustainability
The capacity of the global economy to accommodate enduring demographic, economic, and resource consumption growth remains an enduring issue that regularly raises concerns. Population growth and increased living standards allow individuals access to an extensive array of goods and services. Since the 1970s, many statements and declarations have been made asserting that the world would be unable to sustain such growth without a possible socioeconomic and environmental breakdown. This perspective takes its roots in Malthusianism, which considers the relationships between population and resources as finite. While this has been demonstrated to be inaccurate, since resource availability and the quality of life steadily increased, there are recurring concerns that a threshold could be reached at some point, leading to a breakdown. However, there is limited evidence about this threshold and which environmental and economic conditions would be conducive. A narrative of urgency and impending doom has also emerged, mainly used as a psychological tool to influence public opinion.
Initial environmental actions were related to national regulations over air quality, water protection, waste management, and hazardous materials. These national concerns, particularly in developed economies, were extrapolated as transnational issues encompassing the world and should be addressed. The process began with the United Nations Conference on the Human Environment in 1972, identifying key environmental principles such as natural resources conservation, wildlife protection, and pollution control. It culminated in 1987 with the publication of the Brundtland Report, where the term sustainable development was first formally defined and became mainstream. The concept was further expanded with the United Nations Conference on Environment and Development in 1992, particularly with the setting of Agenda 21, a non-binding action plan for sustainability principles. After a series of iterations, in 2015, the United Nations General Assembly issued a resolution labeled Agenda 2030, that defines 17 sustainable development goals.
As these numerous goals underline, sustainable development is a complex and multidimensional concept subject to interpretation since it involves several scientific disciplines and possible interconnections. Unsurprisingly, the subject is prone to confusion and ideological capture regarding its nature, consequences, and appropriate response. However, it is generally agreed that sustainability favors conditions that benefit the environment, the economy, and society without compromising the welfare of future generations. Still, as history demonstrates, the conditions of future societies largely depended upon the legacy of past societies. All forms of assets (capital, real estate, infrastructures, natural resources, knowledge) passed on to the next generation should be at least of equal value (utility) per capita. What is new is the inclusion of environmental capital, particularly ecosystems, into this perspective. The expansion of this temporal framework into the concept of sustainability includes three major pillars:
- Social equity. Relates to conditions favoring a distribution of resources among the current generation based upon comparative productivity levels and the promotion of equality of opportunities. This implies that individuals, institutions, or corporations are free to pursue their choice and reap the rewards for their risks and efforts. Defining social equity is usually the most challenging element of the concept of sustainability. It should not be confused with equality of outcome (or socialism), where discriminatory practices are implemented in favor of one socioeconomic group and against another with the stated objective of correcting perceived inequalities.
- Economic efficiency. Concerns conditions enabling higher levels of economic efficiency in terms of resource and labor usage. It focuses on capabilities, competitiveness, flexibility in production, and providing goods and services that supply market demand. Under such circumstances, factors of production should be freely allocated, and markets open to trade.
- Environmental responsibility. It involves developing a footprint for human activities, which is lesser than the capacity of the environment to accommodate. This includes the supply of resources (food, water, energy, etc.) and the safe waste disposal. Its core tenets include the conservation and reuse of products and resources.
b. The governance of sustainability
Another important debate relates to the governance of sustainability, such as to what extent public and non-governmental institutions (both at the national and supra-national levels) have a role to play. There are competing approaches, one advocating that sustainability be promoted through regulations and the other that the main driver should be market forces and individual behavior. Environmental advocacy groups are dominantly leaning towards regulations and authoritarianism, perspectives highly influenced by Marxism. They would argue that sustainability is a much too long-term concept to be addressed by corporations or individuals focused on the short term. A counter-argument could be made that the time horizon of governments, especially democratic regimes, is also very short. In rare instances, governments have shown to be proactive regarding environmental matters. Further, special interest groups have captured the decision-making and regulatory apparatus of many governments, implying that environmental policy is influenced by groups representing contradictory ideological perspectives, often based on misleading assumptions.
An emerging perspective concerns influencing investment decisions in infrastructure-dependent sectors such as transportation by promoting a set of standards labeled Environmental, Social, and Governance (ESG) Criteria. A core argument is if the flow of capital investment could be incited to comply, particularly through large financial institutions and pension funds, recipients would become more compliant with sustainability principles. The question remains as if expectations can be placed on rating agencies (and regulators) that seek to enforce compliance or on entities that seek to optimize efficiency and profit (corporations and individuals). Paradoxically, while governments tend to be inflexible and unable to adapt, corporations have demonstrated a resounding ability to shift their strategies and provide products that reflect societal expectations, such as environmentally responsible products. Further, consumer behavior is a key factor in achieving sustainability as it influences the provision and delivery of goods. This complex relationship underlines the respective roles of regulations and innovations in achieving a higher level of sustainability. ESG remains an iterative process of conflicting views and interests based on assumptions that can be presented as certitudes.
c. The geography of sustainability
Societies do not contribute to environmental impacts at the same level. Sustainability can be thus expressed at two spatial levels:
- Global. Concerned with the long-term stability of the earth’s environment and the availability of resources to support human activities.
- Local. Concerned with localized forms of sustainable systems, which are often related to urban areas in terms of jobs, housing, and environmental pollution.
Since a growing share of the global population is urbanized, sustainability has increasingly focused on urban areas, which is not surprising as global impacts are the outcome of local processes and patterns. Major cities require a vast array of supporting infrastructures, including energy, water, sewers, and transport, and a key to urban sustainability issues is linked to their provision and maintenance. Still, cities are context-specific with a unique range of challenges related to their location, pattern, trade system, and level of development. For instance, many cities in developing economies lack basic infrastructure, while their environmental conditions deteriorate due to congestion and motorization. In advanced economies, the quantity and quality of infrastructure are commonly adequate, and their environmental footprint has been decreasing per capita. Thus, there is a geographical divergence in urban sustainability.
Another divergence concerns the ownership and operation of transport infrastructures can be publicly or privately owned, creating a complex governance landscape. Public infrastructures tend to focus on collective passenger mobility and have the advantage of being available to a larger share of the population at a low cost (commonly free of access). Still, they are expensive for the government to maintain (subsidies), which has to rely on alternative models such as public ownership and private operations. Private infrastructures are usually financially profitable and related to individual mobility and freight distribution. As income levels increase, some infrastructure problems are solved, while some environmental problems are created. For instance, an increase in income is linked to better sanitation and water provision, but at the expense of more significant waste generation and carbon emissions. Global sustainability remains influenced by the paradox of a declining environmental impact per capita of several factors, such as energy consumption, carbon emissions, and materials, but also reflects a divergence in the carbon footprint between developed and developing economies.
2. Sustainable Transportation
Transportation, as a core component supporting the interactions and development of socioeconomic systems, has also been the object of much consideration as to what extent it is sustainable.
Sustainable transportation is the capacity to support the mobility needs of a society in a manner that is the least damageable to the environment and does not impair the mobility needs of future generations.
Sustainable development applied to transport systems requires the promotion of linkages between environmental protection, economic efficiency, and social progress. Expected outcomes of sustainable transport include improvements in efficiency, safety, and the environment. Under the environmental dimension, the objective consists in understanding the reciprocal influences of the physical environment and the practices of the industry and that all aspects of the transport industry address environmental issues. Under the economic dimension, the objective consists of promoting economic efficiency by inciting the provision of needed infrastructures and mobility systems. Transport must be cost-effective and capable of adapting to changing demands. Under the social dimension, the objective consists of upgrading living standards and quality of life.
Automobile dependence is a situation that is commonly associated with an unsustainable urban environment. However, such an observation is at odds with the mobility choice and preferences of the global population, where the automobile is rapidly adopted when income levels reach a certain threshold. Other transport alternatives do not measure up to the convenience of the automobile. Automobile dependency is thus the outcome of market forces expressed as consumer preferences, the provision of road infrastructures, and national manufacturing policies. Private and flexible forms of transportation, such as the automobile, are thus fundamental to urban mobility and should not be discarded as options for the sake of ideological perspectives about what should sustainability implies.
Recent advances in car-sharing technologies and the potential for self-driving vehicles underline a much more sustainable usage of car assets that could remove up to 90% of vehicles from the streets. This adds to the ongoing engine and drive technology improvement, reducing vehicle emissions. This contradicts the bias observed in the transport community towards emphasizing public transit and non-motorized transportation as the dominant strategy for sustainable transportation. Yet, almost all public transit systems are financially unsustainable, imposing burdens on society that are accepted because they provide access to all socioeconomic groups. Freight transportation must also be considered in this process, considering the substantial growth of raw materials and goods traded in a global economy. Freight transportation relies more on environmentally sound modes such as rail and maritime transport.
Measures to promote transport sustainability have their limits. Indeed, the built environment, transport infrastructures, and even modes cannot change quickly enough to solve the bulk of the problems related to unsustainable transport. Most investments will remain so for 50 years or more. New investments (in additional or improved infrastructure) will not represent much more than a few percentage points in reducing traffic congestion and its negative externalities. The different life spans of transport modes and infrastructure underline that sustainability cannot be applied in a synchronized fashion. For instance, replacing most of the automobile fleet with more efficient vehicles within a decade could be possible. At the same time, replacing road infrastructure (e.g. pavement) would take about a quarter of a century, and assets such as planes and containerships have a lifespan of a couple of decades.
While policies, rules, and regulations expect compliance, users instinctively react to price signals and discard modes that are becoming costly (unsustainable) and find loopholes. Transportation and sustainability for both passengers and freight must also contend with mitigation versus adaptation issues:
- Mitigation concerns the improvement of productivity and efficiency of existing modes, terminals, and managerial approaches so that environmental externalities are reduced. They tend to be short to medium-term strategies.
- Adaptation is a change in the level of use and the market share of respective modes to reflect better long-term trends, such as higher energy prices, improved information technologies, and stricter environmental regulations.
There is a wide range of environmental sustainability responses, with different local, national, and international regulations. This involves a variety of costs in transport operations that must be built into the price of providing transport facilities and services. Environmental sustainability represents a growing area of responsibility for transport service providers, inciting them to acquire expertise in environmental management. The most important challenge is implementing environmentally sustainable transport within competitive market structures, leaning on coping with changes in transport demand while improving transport supply.
3. Managing Transport Demand
To effectively mitigate the adverse impacts of current transportation systems, strategies can be devised to manage (reduce) transport demand for passengers and freight as well as to redistribute this demand in space or in time (outside peak hours) when possible. Profitable, affordable, and unsubsidized transportation is a good indicator of its sustainability. Increasing transport costs and the pressure to subsidize them can be interpreted as signals that they may be unsustainable. There are several interrelated ways in which transportation systems can adapt to cope with transport demand and reach a better level of sustainability:
- Full-cost pricing. The full (or partial) recovery of costs related to public investments is incurred in constructing, maintaining, and operating transport networks. They remove artificial signals such as subsidies and let users assume the real transportation cost, including road pricing and pollution (carbon) taxes and fees. Motorists are charged a floating fee (depending on demand variability in peak and off-peak hours) for using targeted roads. This can be implemented through various techniques, such as tolls or licensing fees. Tax and pollution fees would involve the implementation of increased taxes on vehicle and fuel purchasing as well as imposing fees on vehicle owners who operate at low levels of energy efficiency. Such an approach aims to incentivize users toward more sustainable mobility choices.
- Parking controls. By raising parking prices or reducing the amount of parking space, such a strategy can deter the use of privately-owned vehicles in areas of highest demand by raising the price of commuting by car to high-density areas. The expected result is to encourage (or force) commuters to seek alternatives in mass transit, ridesharing, or carpooling. They tend to be ineffective for freight distribution since delivery trucks will infringe regulations for short-duration deliveries (e.g. double parking for a few minutes).
- Trip avoidance. A more direct method of reducing traffic demand, but avoiding trips is a complex endeavor. It involves strategies where an activity still occurs while its related mobility is mitigated. This is mostly related to the use of information technologies, which paradoxically can, at the same time, substitute for and support mobility. For instance, e-commerce can reduce the number of shopping trips, but this involves substituting for parcel deliveries. For freight transportation, trip avoidance is mostly the outcome of changes in sourcing strategies such as nearshoring, where fewer ton-km are generated.
- Traffic bans. Through traffic bans, the regulatory institution would exert direct control over the allowable limit of vehicles in a given urban area or along specific corridors depending on measures of transport supply-demand functions or arbitrary estimates of carrying capacity. Many high-density central areas have closed streets to pedestrians to create public spaces more conducive to commercial and social activities.
Implementing such strategies relies heavily on the existing spatial structure, passengers and material flows, and transport networks. An expectation is that the demand will shift towards more carbon neutral modes with better energy performance. In situations where a fee structure is not effective (e.g. low-income population), constraint-based strategies can be more suitable than fee-based strategies. Such coercive strategies would thereby limit the number of vehicles in circulation and, correspondingly, reduce congestion and air pollution while promoting alternative transport means. Their fundamental shortfall is they assume that government (planning) entities know solutions to urban transport problems (such as the appropriate number of parking spaces), which is not necessarily the case.
4. Improving Transport Supply
While implementing demand-oriented policies and mechanisms is important in promoting sustainable transport, these measures can be more effective with transport supply improvements. Transportation infrastructure should be expanded to accommodate rapidly growing transport demands. As long as the global urban population continues to grow, particularly in developing economies, there are pressures to expand urban transport infrastructures and the infrastructure supporting global trade.
In urban areas, the challenge is to expand and improve transportation supply to provide alternatives to the automobile and trucking. This can be achieved for passengers by expanding public transit infrastructure, improving existing public transit services, and making cities friendly to pedestrians and non-motorized vehicles. However, it appears that vehicle automation could be an even more effective tool by allowing better utilization of existing vehicle and road assets as well as reducing the number of vehicles in circulation. The realms of green logistics and city logistics have received renewed attention as tools to improve the sustainability of freight distribution since the material needs of economic activities, including end consumers, must be provided for as well.
Sustainability is giving public transit a new impetus since the bulk of its prior rationale was to mitigate automobile dependency and provide mobility to a large share of the population. However, this is an extremely difficult challenge considering the prominence the automobile is achieving worldwide. It must be acknowledged that this prominence is the outcome of many positive factors favoring the automobile, such as flexibility, convenience, and relatively low ownership and operating costs. As the average income of the global population is increasing, the pressure for automobile ownership continues. Thus, alternatives can be provided if they are cost-effective while fulfilling a niche demand. They may include:
- Energy intensity of vehicles and carbon intensity of fuels. Vehicles are the first element of the transport supply, where more sustainable improvements can be implemented. This is the dimension for which the decarbonization of transportation can lead to the most tangible outcomes. There are many strategies, such as using lighter materials (e.g. composites) for manufacturing vehicles or more efficient or new engine technologies. The material intensity of an average vehicle of 1.5 tons remains significant since steel and plastics can account for 75% of their mass. Because of its complexity and related supply chains, the automobile is subject to circular economy considerations where vehicles, parts, and materials could be reused and recycled. Fuels can also be improved using alternatives such as natural gas, biofuels, electricity, or hydrogen.
- Densification and agglomeration. A higher concentration of activities usually leads to more efficient transportation because of the lesser distances involved. Spatial structures such as logistics zones or transit-oriented developments can thus result in reduced vehicle trips. They may also incite using modes more prone to economies of scale (more passengers or units of cargo per load or surface unit) as cost-effective alternatives. With market signals related to land cost, densification, and agglomeration often dictate more efficient and higher-density uses.
- Context appropriate transport. Transportation modes and infrastructure must be developed and used in the context in which they are the most appropriate. However, the relevance of specific transportation systems to service-specific contexts is subject to debate since it is reflective of societal values and priorities. Both public and private forms of transportation have roles to fulfill. The last decades have seen substantial growth in individual mobility despite all the efforts to promote public transportation. In the North American context, promoting public transit has seen limited success. Therefore, public transportation, being less flexible, should assert a complementary role. The expansion and development of mass transit systems must make effective use of urban space by conforming to a number of factors, including urban form, density, and modal preferences. In doing so, the fleets and networks must ensure a level of flexibility while ensuring low ridership costs. Comparatively, improving and upgrading existing public transit services should include improving service coverage and quality and increasing frequency where and when it is most needed (during peak hours). A similar observation applies to freight distribution, as a range of modes is available to accommodate a variety of supply chains. There is not necessarily an ideal setting in which a mode should be used.
- Micromobility. Integrating individual modes of non-motorized transport, such as walking, electric scooters, and cycling, can provide access to shopping, schools, and work. The main constraint concerns range and capacity as micromobility is not designed to accommodate trips of more than 5 km, with most of the trips less than 1 km. Also, for cities struggling with serious traffic congestion and air pollution, micromobility should be considered an alternative, or at least complementing, private vehicles while serving as a crucial link in an integrated public transportation system; its last mile. While cycling and scooters can be challenging to promote and integrate into urban transportation (e.g. taxing weather conditions such as winter or excessive heat), there is a clear and unmet need to better integrate pedestrian movements into sound urban design and architecture. For freight, non-motorized transport modes are much more limited in capacity and range.
However, such alternatives contrast with the reality of modal choice towards the automobile and trucks, particularly in economies experiencing rapid growth. Thus, sustainable transportation remains elusive since any economic activity, including transportation, has negative environmental impacts. The matter remains if these activities are taking place at a level exceeding the environmental and social carrying capacity. Technological innovation has historically played a paradoxical role in both exacerbating environmental and sustainability issues and, at the same time, offering forms of mitigation. The expectation is that in the future, technological innovation in the transport sector will be more of a sustainability driver than it was in the past. This is why a share of the attention has shifted toward decarbonizing transportation.
5. The Push for Decarbonization
Decarbonizing transportation aims to reduce, mitigate, and even eliminate carbon emissions by adapting transportation infrastructures, conveyances, and operations.
The concept of sustainable transportation has become widely accepted as a goal and appears in the environmental plans of many governments and corporations. For instance, the European Union has set the ambitious goal that by 2030, net greenhouse gas emissions will be reduced by 55% of their 1990 levels. Still, sustainable transportation remains elusive as it does not offer clear guidelines but mostly a narrative allowing stakeholders to remain vague in their commitments and endeavors (also known as “greenwashing”). In the 17 sustainable development goals identified by the United Nations, transportation was not identified as a distinct sustainability goal, even if it accounts for about a quarter of global CO2 emissions. Further, these goals have no stated priority and are subject to interpretation and ideological capture by advocacy groups. Elite individuals and institutions use the environmental and sustainability narrative for virtue signaling and the derived social status.
Since the early 2000s, sustainability goals in the transportation sector have been reframing towards a more tangible strategy focusing on carbon consumption and emissions. This mainly took shape around the decarbonization of transportation, which helps articulate the narrative around the role of fossil fuels. The concept does not undermine the purpose of transportation, which is providing mobility to passengers and freight, but that the carbon footprint of transportation activities should be reduced. Even if it focuses on carbon, decarbonization directly impacts other externalities as most air pollutants are an outcome of the combustion of fossil fuels. To articulate decarbonization strategies, three scopes of emissions have been proposed:
- Scope 1 (Direct emissions). Carbon emissions (and other greenhouse gases such as nitrogen oxide) the result of the activities of an organization. This particularly concerns emissions from vehicles and facilities supporting operations.
- Scope 2 (Indirect emissions). Emissions resulting from the generation of fuels and electricity for operations. Even electricity, which may not generate Scope 1 emissions, could generate Scope 2 emissions if generated by fossil fuels such as coal and natural gas.
- Scope 3 (Indirect/induced emissions). Emissions resulting from activities upstream and downstream of the organization. They are the most complex to assess as they are not under the direct control of an organization generating Scope 1 and 2 emissions. This includes emissions resulting from business travel by management, the commuting of employees, waste generation and disposal, the goods and services that an organization purchases, and the transportation and distribution services used for procurement and access to markets.
Decarbonizing transportation focuses on three main realms of application:
- Infrastructure. The fixed asset components of decarbonization include transport corridors and terminals. Their construction, maintenance, and upgrade can be subject to procurement strategies that are less carbon-intensive, including the use of materials. Transportation modes, particularly in terms of their economies of scale, can be ranked by carbon intensity. This implies that infrastructure-supporting modes with low carbon intensity and connectivity (intermodalism) between transportation modes should be favored.
- Conveyances and equipment. For mobile transportation assets, the focus is mainly on their fuel and energy sources, including shifting to fuels emitting less CO2. This could also involve a shift to less carbon-intensive modes, but modal shift strategies usually have limited impacts. The electrification of roads and rail is a key strategy as it focuses on modes with the highest contribution to CO2 emissions. Autonomous vehicles have potential since they can provide mobility with fewer vehicles, and their routing can be optimized in real-time to reduce energy consumption. Ideally, pedestrians and bicycles should have a larger share of urban mobility.
- Management and operations. A focus is on pricing strategies that change the competitiveness of transportation modes according to their carbon emission. It also considers an array of regulations concerning issues such as emissions and types of fuels. The expectation is that the increasing competitiveness of decarbonized transportation will displace transportation technologies based on fossil fuels. Better utilization of existing transportation assets is also recognized, such as freight platforms and ride-sharing services.
The push towards the decarbonization of transportation mostly concerns electrification, but modes like air and maritime will switch to alternative fuels such as LNG and ammonia. Electric vehicle sales are rising rapidly, accounting for 14% of global sales as of 2022. They accounted for about 5% of all vehicle sales in the United States, 7% in China, and around 15% for most European countries (above 70% in Norway). Electrification represents a temporary devolution of mobility as fundamental attributes such as range (battery charge) decline. It thus represents a decline in flexibility and operational performance. This transition will likely continue until electric vehicles perform similarly to their internal combustion engine equivalent. Sustainability and decarbonization are part of the same agenda, with the main difference being that decarbonization offers a clearer framework and plan of action with practical solutions.
- 4.1 – Transportation and Energy
- 4.3 – The Environmental Footprint of Transportation
- 3.1 – Transportation and Economic Development
- B.15 – Green Logistics
- City Logistics
- A.20 – Transport and Environmental Management
- Banister, D. (2008) “The Sustainable Mobility Paradigm”, Transport Policy, Vol. 15, No. 2, pp. 73-80.
- Banister, D. and K. Button (eds) (1993) Transport, the Environment, and Sustainable Development. London: Spon Press.
- Black, W.R. (2010) Sustainable Transportation: Problems and Solutions, New York: The Guilford Press.
- Ellen MacArthur Foundation (2012) Towards the Circular Economy Vol. 1: an economic and business rationale for an accelerated transition.
- Gilbert, R. and A. Perl (2008) Transport revolutions. Moving people and freight without oil, London: Earthscan.
- Haas, T. (ed) (2012) Sustainable Urbanism and Beyond: Rethinking Cities for the Future. New York: Rizzoli.
- Hickman, R., P. Hall and D. Banister (2013) “Planning more for sustainable mobility”, Journal of Transport Geography, Vol. 33, pp. 210-219.
- Humphreys, R. M. and A. Dumitrescu (2021) Decarbonizing the Freight and Logistics Sector. Transport Decarbonization Investment Series. World Bank, Washington.
- IEA/OECD (2009) Transport, Energy and CO2: Moving toward sustainability. Paris: International Energy Agency.
- International Transport Forum (2020) Transport Climate Action Directory, Paris: OECD.
- Lacy P., J. Long, and W. Spindler (2020) The Circular Economy Handbook: Realizing the Circular Advantage, London: Palgrave Macmillan.
- McKinnon, A. (2018) “Balancing Efficiency and Resilience in Multimodal Supply Chains”, International Transport Forum Discussion Papers, Paris: OECD Publishing.
- McKinnon, A. (2018) Decarbonizing Logistics: Distributing Goods in a Low Carbon World, London: Kogan Page.
- McKinnon, A., M. Browne and A. Whiteing (eds) (2013) Green Logistics: Improving the Environmental Sustainability of Logistics, Second Edition, London: Kogan Page.
- Newman, P. and J.R. Kenworthy (1999) Sustainability and Cities: Overcoming Automobile Dependence, New York: Island Press.
- Noussan, M., M. Hafner and S. Tagliapietra (2020) The Future of Transport Between Digitalization and Decarbonization: Trends, Strategies and Effects on Energy Consumption. SpringerBriefs in Energy.
- Schiller, P.L., and J.R. Kenworthy (2018) An Introduction to Sustainable Transportation: Policy, Planning and Implementation, New York: Routledge.
- Tolley, R. (ed) (2003) Sustainable Transport: Planning for Walking and Cycling in Urban Environments, New York: CRC Press.
- UN-HABITAT (2009) Planning Sustainable Cities, Global Report on Human Settlements 2009, United Nations Human Settlements Programme, London: Earthscan.
- United Nations (2021) Sustainable transport, sustainable development. Interagency report for second Global Sustainable Transport Conference.