1 Introduction

With a population of nearly 700 million, rapid economic growth, and continuing population growth, the ASEAN countries are pivotal for the global climate mitigation effort. The transportation sector accounts for about 25% of global CO2 emissions [1], of which three quarters come from road transportation [2]. In the ASEAN countries, road transportation ranks as the second largest source of energy-related emissions [3]. The evolution of the transportation sector in the ASEAN countries is therefore also important for the rest of the world.

Expanding public transportation, such as buses, trains, subways, and light rail, is a climate mitigation strategy with significant co-benefits that can help address important urbanization and development challenges. The co-benefits include improved air quality, reduced noise and congestion, and improved road safety. Public transportation also facilitates equitable development because it increases access to education, work, and services, while also reducing traffic fatalities and air pollution—issues that disproportionately affect low-income groups [4].

In the ASEAN countries, expanding public transportation also offers some advantages over two other climate mitigation measures: phasing out coal and introducing electric vehicles (EVs). Phasing out coal is a key climate measure but it is also the most difficult one for the ASEAN countries due to the entrenched interests of the coal sector and its abundance in ASEAN’s largest member state, Indonesia [5,6,7]. By contrast, public transportation impacts fewer stakeholders negatively, rendering it a more politically and economically feasible mitigation measure to implement relatively quickly. Public transportation expansion also holds a competitive advantage over adopting privately owned electric cars, especially as the electricity mix in the ASEAN countries remains heavily reliant on coal and natural gas [8]. The economic feasibility of public transportation projects, however, still depends on the type of public transportation and the context of each project.

With a few exceptions, mobility in the ASEAN countries is hampered by a lack of access to public transportation services [9] and as a result, the region’s megacities struggle with some of the world’s worst congestion. The Manila metropolitan area has been identified as the world’s most congested city, while Bangkok, Jakarta, and Kuala Lumpur are not much better [10]. However, the congestion is not limited to the region’s megacities; in second-tier Indonesian cities such as Malang and Padang, residents spend close to 25% of their driving time in traffic jams—similar to famously congested Jakarta [11].

The traffic in many ASEAN cities not only causes frustration over time lost in traffic but also contributes to high levels of air pollution and lower economic productivity. Road congestion alone is estimated to cause an annual economic loss of 2–5% of GDP in most Asian economies [12]. Transportation is also responsible for 80% of urban air pollution in Asia, with all major ASEAN countries except the Philippines included among the 40 countries in the world with the worst air pollution [13]. Consequently, air pollution is a major cause of premature death in the region and the economic cost of inaction on air pollution is estimated to be two to three times higher than the cost of implementing measures to limit it (see Table 3 in the Appendix).

Lastly, time inconsistency—the tendency of policymakers and individuals to prioritize short-term benefits over long-term gains—can make policies with delayed payoffs harder to sustain [14]. This is less of an issue for public transportation policy than for some other GHG reduction measures. Because improvements in public transportation can yield relatively quick and broadly shared benefits, they are more tangible in people’s daily lives and less susceptible to the issue of time inconsistency. This immediacy makes the expansion of public transportation a climate mitigation measure with the potential to rally public and electoral support. However, the popular support for these solutions still hinges on securing equitable access, voice for communities affected by developments, avoiding overcrowding, and maintaining passenger safety.

This study makes two main contributions to research, one thematic/policy-oriented, the other empirical/methodological. The first is to spotlight public transportation in the ASEAN region, which has received little cross-country attention in the existing climate policy research, with only a few academically published contributions [15,16,17,18] and some grey literature [11, 19, 20]. There are more works on individual countries, cities, and projects in Asia, but any attempt to compare the accessibility of public transportation and the modal split of transportation across countries in the region runs into problems with both data availability and comparability. (“Modal split” refers to the distribution of travelers or trips among different modes of transport, such as cars, buses, trains, or walking.) When information is available on the accessibility of public transportation and the modal split within a city, it is still difficult to compare them because of variations in definitions, data gathering methodologies, and operationalizations among government census agencies and other institutions that gather primary data. The ASEAN-Japan Transport Partnership offers some cross-country standardization in its statistics on road transportation, but many data are missing. This suggests a need for more harmonized data collection efforts to support comparison and policy development.

Our second contribution to research is empirical and methodological and consists in the introduction of NDCs as the empirical basis for studying the prioritization of public transportation expansion among other climate mitigation measures. See Table 4 in the Appendix for an overview of existing studies that take NDCs as their empirical basis—most of which focus on topics other than public transportation.

In the next section, we lay out our methodological approach to the analysis of NDCs. The third and fourth sections present the results and discussion of our analysis, and the final section concludes and makes recommendations for policymakers and further research.

2 Methods

There are several reasons why we use the NDCs as our source of data: the NDCs are official, overarching policy documents where countries showcase their climate policies for both domestic and international audiences. The policies presented in the NDCs are thus backed up by highly public commitments. It would be paradoxical if a country had an important climate policy element to which it was committed, but did not include it in its NDC. Finally, the NDCs provide for a degree of empirical consistency, as they are all created within the framework of the Paris Agreement. However, at the same time, each state unilaterally determines the content of its NDCs [21]. This means that the NDCs are notoriously hard to compare, owing to the great variation in their scope, coverage, target types, baseline years, and mitigation scenarios [22,23,24,25,26,27]. We attempt to overcome this challenge by focusing on targets and measures within the mitigation sections of the NDCs, enabling us to develop quantifiable proxies for the prioritization of transportation relative to other areas of climate mitigation.

We analyze the sectoral mitigation targets and measures in the mitigation component of the ASEAN countries’ active NDCs (as of February 2025). Monitoring provisions, such as ways to facilitate reporting on progress towards targets, have not been counted as “measures” for the purpose of this study.

We developed a coding scheme where targets and measures are coded according to sectoral categories in the mitigation component of the NDCs. Instead of counting keyword search hits, we did a close reading of the NDCs, which is an important quality control to capture the context and determine the level of detail and specificity. Taking this approach allowed us to identify subsectors and the conditionality type of each target and measure (or the lack thereof). Our approach is similar to that of other studies analyzing specific topics in the NDCs [28,29,30,31,32]. For further details on our approach, see the codebook in the Supplementary Material [33].

All the ASEAN NDCs include some sectoral information, but in different ways and with varying levels of detail. Transportation is often mentioned as a subsection of energy but sometimes it is classified as a separate sector. However, four out of the ten ASEAN countries do not include additional information on sector-specific or sub-sectoral mitigation targets and measures and instead list the sectors which will contribute to achieving their headline mitigation targets (Malaysia, Philippines, Singapore, Thailand). We counted these as “mentions” and included them in the overall number of about 400 sectoral targets, measures and mentions serving as the basis for our analysis.

This approach of counting and comparing the number of targets, measures, and mentions does not in and of itself say much about the quality of the target or the measure—however, when comparing with other sectors it can still serve as a rough proxy for how much relative focus the countries are giving to public transportation in their NDCs.

Once we had compiled the database of targets, measures, and mentions, we analyzed it using the Avoid-Shift-Improve (ASI) framework for sustainable transportation. ASI provides a framework for analysis of attempts to reduce the environmental impact of transportation through the reduction of GHG emissions, energy consumption, and congestion.

ASI divides policy options for reducing the environmental impact of transportation into three categories: 1) measures to avoid the need to travel or reduce the distance travelled (Avoid), 2) measures to shift travel from high- to low-emission modes (Shift), and 3) measures to improve the energy efficiency of vehicles or make fuels less carbon-intensive (Improve) [34]. To apply the ASI framework to our dataset, we examined the nature and intent of each of the 34 transportation-related policy measures and targets in detail to determine whether they fell under the Avoid, Shift, or Improve category.

ASI was developed in Germany in the early 1990s [35], and has since been used in a number of academic works [34, 36,37,38,39,40,41,42]. In addition to the role of transportation in mitigating climate change, many ASEAN cities face a complex set of additional urban development challenges. By using the ASI framework to classify policies included in the NDCs, we aim to better understand how the NDCs align with sustainable transportation options, urban development, and socioeconomic priorities.

3 Results

In our data, public transportation accounts for only 2.2% of the total mitigation targets, measures, and mentions in the NDCs of the ASEAN countries. These public transportation targets are not spread evenly across the countries and are present only in the NDCs of Cambodia, Indonesia, Laos, and Vietnam (see Table 2). A first finding is thus that the NDCs of the other six ASEAN countries—Brunei, Malaysia, Myanmar, Philippines, Singapore, Thailand—lack any public transportation elements whatsoever in their mitigation sections.

As shown in Table 1, the energy sector dominates the NDCs of the ASEAN countries, representing more than a third of the total number of targets, measures, and mentions. Land-use, land-use change, and forestry (LULUCF) is the second largest sector, followed by agriculture, and waste. The transportation sector as a whole, including not only public transportation but all transportation-related climate measures, accounts for 8.4% of the total number of sectoral targets, measures, and mentions. Within the transportation sector, nearly half of the targets, measures, and mentions concern EVs (16 out of 34). The second largest category is public transportation at 9, followed by biofuels and energy efficiency measures for transportation (see Table 1).

Table 1 Targets, measures, and mentions by sector in the ASEAN countries’ NDCs
Full size table

Applying the Avoid-Shift-Improve (ASI) framework to public transportation in the NDCs of the ASEAN countries, we found that of the nine targets and measures, seven belong to the “Shift” category and two belong to the “Improve” category, while none are in the “Avoid” category (see Table 2).

Table 2 Public transportation targets in the ASEAN NDCs classified according to the ASI framework
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We see that for Cambodia, Laos, and Vietnam, all public transportation targets and measures are about increasing the modal share of public transportation and thus belong to the “Shift” category. The two points on public transportation in the Indonesian NDC, however, focus on fuel efficiency measures in already existing public transportation systems, and accordingly belong to the “Improve” rather than the “Shift” category.

Zooming out and applying the ASI framework to the full 34 transportation sector targets, measures, and mentions in the NDCs of the ASEAN countries, we can see that they mostly belong in the “Improve” category, followed by the “Shift” category, while we found none belonging in the “Avoid” category.

When we consider whether the public transportation mitigation targets, measures, and mentions are conditional (dependent on external support) or unconditional (a country is ready to undertake the action on its own), we find that seven out of nine are unconditional. Figure 1 presents the percentage of total transportation sector targets and measures in the ASEAN NDCs that are conditional, unconditional, or of unspecified conditionality, and indicates the respective percentages represented by public transportation. Figure 1 also shows that there are more unconditional than conditional measures for public transportation than for other sectors.

Fig. 1
Fig. 1
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Transportation subsector targets, measures, and mentions by conditionality type

This balance in favor of unconditional targets and measures could suggest a willingness on the part of governments to expand public transportation without relying on external funding or support, and that it might be a priority. However, the lack of conditional measures could also be a missed opportunity to attract international financing and technical assistance for public transportation infrastructure, as the conditional parts of an NDC can serve as signals to peers, development banks, and investors about areas where governments would welcome support or collaboration. Although much public transportation technology can be considered mature compared to some other climate solutions such as blue hydrogen, carbon capture and storage (CCS), or small and medium-sized reactors (SMRs), large infrastructure developments often require significant capital expenditure and know-how, and conditional targets could be a way to open up for such investment and knowledge-sharing. This is especially relevant for large elevated or below-ground rail systems within cities and high-speed rail between cities, which are the most demanding types of public transportation. Balancing unconditional targets with conditional targets could potentially bring in more capital and technology and unlock more robust and scalable progress in public transportation projects.

3.1 Cases

To exemplify some of the nuances and range of challenges and opportunities involved in expanding public transport in the ASEAN countries, we present five brief, non-systematic micro-cases here.

3.1.1 Bangkok

The Bangkok Metropolitan Administration’s project to replace old diesel buses with electric ones has experienced delays. However, in 2025, 500 battery-electric buses were to be deployed [43]. This was a first step toward retiring 2 300 combustion engine buses (60% of the fleet) by 2029. A study of two of the routes proposed for electrification in Bangkok find that electric buses are cheaper than diesel buses both in operation and in upfront investment: THB 10.4 million lower annual energy costs and 37% lower upfront investment compared to diesel [44]. A study from India indicates that urban diesel buses emit approximately 65–76% less CO2 per passenger kilometer than privately owned battery-electric cars, while battery-electric buses cut another 24% compared to electric two- and three-wheelers and 71% compared to private cars. This leads the authors of that study to recommend a twin strategy of shifting users from private vehicles to public transport and at the same time electrifying buses [45]. These recommendations may be relevant for Bangkok and other ASEAN megacities, depending on the local electricity mix.

3.1.2 Ho Chi Minh City (HCMC)

After years of delay, Line 1 of the HCMC metro opened in December 2024. The almost 20 km long electric line between the central business district and the growing eastern suburbs exemplifies how shifting travelers to high-capacity public transport can yield emissions cuts, reduce air pollution, and reduce health related issues, according to a study from Vietnam National University HCMC [46]. Japan financed 88% of the project and construction was carried out in partnership with Vietnamese firms, with another six lines planned for the coming years [47]. When ridership is sufficient, as in HCMC, metro projects can make a significant contribution to reducing GHG emissions as well as sustainable urban development. They reduce congestion even more than buses or trams because they remove some traffic entirely from the surface of a city. However, some ASEAN cities like Bangkok and Jakarta are located on swampland and/or are affected by rising sea levels and regular flooding. This makes subterranean metro systems more difficult and expensive in those locations and are some of the reasons why they have instead opted for urban light rail systems that are mostly elevated.

3.1.3 Jakarta

With a population of nearly 33 million, the greater Jakarta urban conglomeration suffers from severe traffic congestion, air pollution, and lack of transport for the poor. Its main public transport system is a network of bus lanes with elevated stops. These work well because most of the bus lanes have physical barriers to keep out other vehicles. However, the extent of this network does not match the population of Jakarta and the stops are few and far between. Further expansion is difficult because the addition of bus lanes eats up space used for the already heavily congested traffic. Most of the non-public transportation is by motorbike, forcing inhabitants to spend large amounts of time breathing in the polluted air, enduring high noise levels and heat, and risking accidents. After many years of discussion and delay, the Jakarta Mass Rapid Transit (MRT) elevated light rail system is being developed. It was proposed in 1985 and the first line opened in 2019, with a limited reach. The start of construction actually led to a substantial worsening of congestion in the city as many main roads became construction sites for several years. Several more lines will need to be added for the Jakarta MRT to fulfill its potential to alleviate congestion and reduce pollution. One important lesson that can be learned from the Jakarta MRT is that it is important to start the development of public transport infrastructure as early as possible. The later one starts, the greater the pain before the situation gets better again.

3.1.4 Manila

The greater Manila Area (GMA) has some similarities with Jakarta: a population of around 27 million with insufficient public transportation, high levels of congestion and air pollution. A special characteristic of Manila is the role of the iconic Jeepneys. These are bus-like vehicles which were originally created from jeeps left by the American military after World War II. They are cramped, noisy, emit clouds of dark exhaust, and are often open air, exposing passengers to the polluted air and noise. Attempts to modernize the fleet of jeepneys has led to resistance from drivers and owners who fear that their livelihoods are threatened. The case of the jeepneys illustrates how not all public transportation is environmentally friendly as well as how changes in transportation systems need to be well thought out and managed to avoid disruption and conflict, as such systems are entangled in complicated ways with the lives of many people.

3.1.5 Singapore

Singapore stands out with 99% of its urban population enjoying easy access to public transportation [48]. Singapore has one of the best public transportation systems in the world, with well-designed infrastructure for pedestrians that connects with public transport stops, maximizing the accessibility of rail and bus infrastructure [49]. This contrasts with some other ASEAN cities, such as the Indonesian city of Yogyakarta, where pedestrian-oriented upgrades along the famous Malioboro Street and other corridors improved walkability from 2018 onwards, yet the networks remain isolated and traffic congestion continues to grow [50]. This shows how well-connected walking infrastructure such as in Singapore is essential to unlock the full mitigation potential of urban public transport. Among the ASEAN NDCs, it is only the Laotian one that mentions non-motorized aspects of their public transportation ambitions.

4 Discussion

A well-organized public transportation system can deliver relatively quick and broadly shared benefits to a city. It is necessary, however, to invest in public transportation at an early point in time, in order to avoid locking in future emissions. The lifespan of a car is 10–20 years and highways can last for centuries, and this longevity of vehicle fleets and transportation infrastructure locks in future emissions [51].

Our analysis reveals that most of the general transportation sector targets, measures, and mentions in the ASEAN NDCs aim to improve modes of travel, with only a few aimed at shifting travel to public transportation, and none to avoid unnecessary travel. Looking specifically at the public transportation targets and measures, most aim at shifting practices, whereas a few seek to improving them. In the rest of this sub-section, we explore our findings further in relation to the ASI framework.

Existing research on sustainable transportation in NDCs finds that “Avoid” measures are underutilized around the world [52]. This is also the case in the ASEAN NDCs, which contain no targets or measures to avoid the need to travel. With the COVID-19 pandemic, work from home (WFH) solutions turned out to be an effective “Avoid” measure [53,54,55]. However, many organizations have rolled back WFH, and some workplaces have implemented return-to-office mandates [56, 57].

Moving on to the “Shift” strategy for developing sustainable mobility solutions, we find a few targets and measures seeking to shift travel to public transportation in the NDCs of Cambodia, Laos, and Vietnam. Another part of a “Shift” strategy for improving sustainability is to increase the share of people walking and cycling. Despite the benefits of combining non-motorized transport (NMT) and public transport measures in terms of emissions reduction and road safety [58, 59] we find only one reference in the mitigation components of the NDCs to making cities more walkable or cyclable: the Laotian NDC prescribes the development of a new bus-based rapid transit system in Vientiane, with an associated NMT-component. The NDC does not elaborate on this point, but NMT usually refers to walking and cycling [58]. A common argument against the viability of walking or cycling in ASEAN is the hot and humid tropical climate — a challenge that is further amplified in urban settings by the heat-island effect, an increase in the near-surface air temperature in cities relative to adjacent rural areas [60, 61]. Expanding urban green infrastructure through tree planting, park construction and similar interventions delivers multiple co-benefits, one of which is a reduction in air temperatures [62]. A study conducted in Kuala Lumpur found that planting trees with a dense canopy along roads reduced the average temperature by as much as 35% [63]. Heat has also been found to constitute less of a barrier to NMT than factors such as safety and convenience [64]. Improving walking and cycling infrastructure and promoting e-bikes and cycling for transportation have been highlighted as ways to increase non-motorized transportation in ASEAN, with potential positive health benefits [64, 65].

The final strategy in the ASI-framework is also the one we find most instances of in our analysis. The “Improve” strategies for the transportation sector identified in the NDCs focus on investments in EVs, biofuels, and other fuel efficiency measures. When it comes to EVs, the current ASEAN electricity mix means that EVs are not as clean as they could be if the electricity mix were renewable. Additionally, most public transportation has a lower carbon footprint per passenger-mile than private vehicles. While the exact difference in emissions levels depend on factors such as energy use, the carbon intensity of the energy source, and passenger load, the ability of public transport to move many people at once underpins its advantage over private EVs as a greenhouse gas reduction strategy. This is why the IEA’s first recommendation for moving the transportation sector towards net zero is to plan for compact urban development with improved public transportation [66].

First- and second-generation biofuels can deliver emission reductions only when the associated land-use change (LUC) avoids large net carbon releases. Although some studies find that biofuels may reduce greenhouse gas emissions, this benefit sometimes comes with trade-offs, including negative impacts on biodiversity, acidification, eutrophication, as well as a large water footprint [67]. Options such as cultivating biofuel crops on restored degraded pastures, cultivating land that used to be barren, or introducing second-season crops for biofuels can sometimes be neutral or even climate-positive. However, at present little biofuel is of such provenance. Third-generation biofuels are not considered effective in lowering emissions, as their production tends to be more emissions-intensive than that of fossil fuels [67].

Lastly, other types of fuel efficiency measures found in the ASEAN NDCs depend on switching out one carbon intensive fuel for another, slightly less carbon intensive fuel. Public transportation is usually thought of as a “Shift” measure, but in the case of Indonesia’s public transportation target in this analysis, it is rather an “Improve” measure as it aims to introduce compressed natural gas (CNG) in public transport. Although such “Improve” measures could somewhat help improve air quality and emission intensity per passenger kilometer, they would not help much with the other challenges posed by the rapid urbanization in the ASEAN region if the modal share of public transportation is not increased.

4.1 Integrating public transportation into the NDCs

The projected slow growth in public transportation in the ASEAN countries is incompatible with the Paris Agreement’s 1.5 and 2 °C temperature targets [68], and the NDCs largely lack public transportation targets and measures. However, many of the ASEAN NDCs do include references to national or regional policy plans for the transportation sector outside of the NDCs. The transportation targets and measures contained in these plans, as well as national-level urban development plans, could be carried over to the NDCs. This is because an NDC is a tool that can help countries with implementation, in several ways.

Wang-Helmreich et al. [51] offer recommendations for reducing emissions in the transportation sector and highlight the importance of international support and cooperation. Focusing more on public transportation in the NDCs could help align actors within and across countries, foster transparency and accountability, and stimulate further bi- and multilateral cooperation on sustainable transportation solutions. More attention to public transportation in the ASEAN NDCs could also help improve the prospect for implementation by mobilizing finance and drawing attention to the importance of public transportation.

It has been estimated that the global public transportation capacity needs to double by 2030 in order to attain the temperature goal of the Paris Agreement [68]. In the ASEAN countries, several of which start from a low level of public transportation, it needs to grow even more. Developing public transportation in the ASEAN countries is also important in the context of what has been referred to as the ASEAN climate and energy paradox [5]: the ASEAN countries will be more affected by climate change than most countries in the world [69], but at the same time they are doing less to combat it than most countries—also when controlled for economic wealth and government effectiveness [70]. This relative underperformance carries several risks: heightened risk of physical impacts of climate change, stranded assets, reduced GDP, and, ultimately, political risk [71, 72]. Even if countries around the world fulfill their current climate pledges, which is not certain, anthropogenic global warming will reach 2.0–2.6 °C and the ASEAN countries stand to lose 17–29% of GDP by 2050 as a result [73, 74].

5 Conclusion

Expansion of public transportation is one of the most effective and affordable climate mitigation measures and has many co-benefits and positive externalities that are particularly relevant for the ASEAN countries: reduced congestion, air pollution, inequity, and traffic accidents. We therefore recommend that the NDCs of the ASEAN countries incorporate robust public transportation targets and measures. Integrating public transportation targets into the next round of NDC updates, both as unconditional and conditional commitments, could help mobilize finance and highlight the need to develop public transportation. It could also align actors across the ASEAN countries, establish norms, foster transparency and accountability, facilitate more consistent and accessible statistics to ease the monitoring of public transportation targets and measures, and stimulate bi- and multilateral cooperation on sustainable transportation solutions. ASEAN’s arenas for cooperation, such as the ASEAN Smart Cities Network for Sustainable Urban Development, could be more consistently utilized to enhance public transportation ambition and implementation across the region.

Most of the transportation targets and measures currently included in the ASEAN NDCs are related to “private” EVs for clarity, biofuels, and other fuel efficiency measures and thus fall within the “Improve” category of the Avoid-Shift-Improve framework. “Private” EVs for clarity running on coal power improve air quality in urban centers and have a (slightly) lower carbon footprint than a “private” combustion engine vehicle per passenger mile travelled, but they do not limit carbon emissions from road transportation nearly enough to fulfill the Paris Agreement. The same applies to fuel efficiency measures, and the sustainability of most biofuels is poor and sometimes terrible. In addition, such “Improve” strategies do not contribute to solving the severe congestion in many ASEAN cities, nor does it contribute to equitable development by improving mobility for the poor.

For the ASEAN countries to fulfil their public transportation potential, more harmonized data collection is needed to facilitate comparative analysis and policy development. Our review of the existing literature identified two primary obstacles to comparing public transportation systems across ASEAN: data availability and data comparability. Information on the availability of public transportation and modal shares is often incomplete, and the data that do exist are unsuitable for purposes of comparison due to variations in data collection methodologies and divergent operationalization of concepts across countries and institutions.

Assessing the scope and impact of a public transport measure set forth in an NDC depends on the extent and quality of the information accompanying the measure. NDCs often lack sufficient detail to analyze the scope or impact of proposed actions. Conducting analysis therefore often requires numerous assumptions to compensate for the lack of data. However, more comprehensive information may be available in national policy plans. A potential direction for future research is therefore to systematically identify and review national public transportation policies. Developing an overview of such policies, particularly for groups of countries that are already engaged in regional cooperation, could help identify gaps and shortcomings and uncover opportunities for enhanced cooperation on public transport. Furthermore, increased attention to public transportation could create a momentum for integrating it as an central sectoral target in the NDCs.

Another possible direction for further research is to expand the analysis to cover all NDCs worldwide. This will require a large research project, as the analysis of each NDC is time consuming and raises many issues.