A scenario analysis of the energy transition in Japan’s road transportation sector based on the LEAP model

Japan has lagged behind other developed nations in transitioning its transportation sector to sustainable energy sources. This study employs the Low Emissions Analysis Platform model to examine six scenarios, assessing energy consumption and emissions associated with four major energy sources and pollutants. Our findings reveal an overall decline in total energy consumption across all scenarios. Notably, the Combined scenario where multiple policies are integrated demonstrates the most significant reduction, with a 56% decrease compared to the Business as usual scenario by 2050. The analysis also indicates that the electricity and hydrogen demand for electric vehicles and fuel cell vehicles remains economically viable within future strategic plans. Emissions, including CO2, Carbon Monoxide (CO), Methane (CH4), and Nitrous Oxide (N2O), exhibit substantial reductions, particularly under the Active Promotion Scenario, where a high EV adoption rate is achieved. Moreover, the Combined scenario resulting in a comprehensive and integrated approach, leads to a remarkable 66% decrease in emissions. These results serve as valuable reference points for the Japanese government, aiding in the formulation of future targets for widespread EV adoption and emission standards for pollutants.


Introduction
One of the biggest obstacles to sustainable development has been the transportation sector [1] which relies on fossil fuels.In 2019, Japan's transportation sector was responsible for emitting 206 million tons of CO 2 , making up 18.6% of the total national emissions.The majority of these emissions, approximately 177 million tons (86%), originated from road transport [2].According to data from the International Energy Agency, global carbon dioxide emissions reached 33 billion tons in 2022, with the transportation sector accounting for 8.2 billion tons, or 24.8% of the total emissions [3].The carbon emissions from the transportation sector in most developed countries are proportionally higher than those in developing countries, indicating that the share of energy consumption and carbon emissions from the transportation sector may continue to grow in developing countries as they evolve into developed countries in the future [4].However, reducing carbon emissions from the transportation sector is relatively challenging, and the pace of carbon reduction may lag behind other sectors by more than 10 years [5,6].Currently, the energy source for the road transportation sector primarily relies on fossil fuels.Although the market for new energy vehicles which include fuel cell vehicles (FCVs), electric vehicles (EVs), and plugin hybrid electric vehicles (PHEVs) have grown in importance within the automobile industry and now entered a phase of rapid expansion, there are still non negligible emissions in the life cycle electricity generation and hydrogen production processes globally [7,8].The development of policies and strategies to support the energy transition in the transportation sector is critical to the success of the transition [9].Many studies have examined the potential of different policy approaches to support the transition, such as subsidies for EVs [10][11][12], carbon pricing [13][14][15], development of public transportation [16][17][18], and regulations on vehicle emissions [19,20].
Japan is the fourth largest economy in the world and has a population of over 120 million people.According to MILT's statistics, Japan's national vehicle fleet has reached over 82 million as of the end of February 2023.However, despite being a key player in the global automotive industry and the secondlargest exporter of automobiles worldwide, Japan's current policies in promoting the electrification of automobiles still lag behind the leading countries such as China and the United States, leading to the overall penetration rate of EVs in Japan much lower than other leading countries.For instance, Japan's incentives for EVs, such as tax credits and subsidies, are considered relatively modest with requirements regarding factors such as weight and range [21], and the low prevalence of charging infrastructure further diminishes consumers' desire to purchase electric EV [22].According to data provided by the Japan Next Generation Vehicle Promotion Center, as of 2021, the total number of battery electric vehicles (BEVs) in Japan was just over 150 000, while PHEVs accounted for over 170 000.The number of FCVs, which the Japanese government and relevant industries place great importance on, was only around 7000.Overall, the penetration rate of electric vehicles in Japan stands at a mere 0.4% [23].Although Japanese automakers have a substantial technological reserve in hybrid electric vehicles (HEVs) and the number of HEVs currently surpasses 13 million, however, the global EVs industry is gradually transitioning towards EVs and PHEVs, with China, the world's largest market for EVs, particularly surpassing HEVs in terms of PHEV adoption.
The energy transition in the transportation sector is essential for achieving carbon neutrality goals, and the Japanese government is currently taking measures such as subsidies and improving infrastructure to further promote the widespread adoption of EVs in Japan, however, there is a current lack of systematic research on the energy transition in Japan's transportation sector.Some researchers have studied the policies in terms of EV and FCV, proposing that the construction of a new societal technological system is necessary from both the perspectives of technological innovation and social transformation [24,25].Some scholars have investigated scenarios for achieving Japan's 2050 carbon neutrality goal considering the overall sectors, including the transportation sector [26], and the potential of introducing electricity and hydrogen to the transportation sector [27].Nevertheless, the extent to which this transition goal can contribute and the impact it will have on the existing energy supply-demand relationship remain to be analyzed with the specification of the transportation sector in Japan.Besides, there is currently not enough research in terms of various scenarios based on multiple policy backgrounds, especially under scenarios with different EV penetration rates, how the energy consumption and emissions of the transportation sector will respond remains a key consideration.
Note that in this research, the boundary of the term 'road transportation sector' of Japan refers specifically to the road vehicles, unless otherwise specified.It does not include railways, aviation, maritime transportation, or other modes of transportation.

Materials and methods
This research used the Low Emissions Analysis Platform (LEAP) model to facilitate energy policy analysis and climate change mitigation assessment.LEAP's broad user base spans government agencies, academics, non-governmental organizations, consulting companies, and energy utilities, with adoption in more than 190 countries worldwide [28].

Model framework
The transportation module of the LEAP model was used in the simulation and the framework is shown in figure 1. Due to the stay-home policies introduced with the COVID-19 pandemic in 2020, the transportation sector has been significantly impacted accordingly.In this study, we selected 2019 as the baseline year and 2021 as the year when the policies took effect, to minimize the influence of stay-home policies on the model results.

Data description
The data used in the LEAP model to simulate the energy consumption and pollutant emissions of the transportation sector in Japan have five parts and are explained as follows:

Annual transportation volume
The official annual passenger-kilometer (p-km) and ton-kilometer (t-km) transported data from 1995 to 2019 comes from The Automobile Transport Statistics Survey [29] conducted by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) of Japan.The historical transportation data is shown in figure 2. Note that the survey method changed in October 2010 and thus the data is adjusted by a connection coefficient (table S1) set by MLIT to ensure the continuity of the figures.The annual transportation volume from 2007 to 2009 was calculated using both the old and new survey methods applied before and after October 2010.The connection coefficients were then set by averaging the ratios of each fiscal year between the two survey methods.

Classification of vehicle type
Vehicles in Japan are divided into freight vehicles and passenger vehicles based on their transport objectives and commercial or private depending on their use.According to the size of the vehicle, they can  be further divided into normal, compact, and light vehicles.Furthermore, in this research, we also considered vehicles in terms of different energy sources including internal combustion engine (ICE) vehicles, HEV, EV, and FCV.The detailed classification and ratio of each type are shown in table S2.Since the official data on transportation excludes different energy sources and thus, we used the ratio of the current stock of vehicles for different energy sources as the transportation ratio.

Fuel efficiency
Fuel efficiency is a rating of how far a vehicle can travel on a specific amount of energy.The less energy the vehicle uses, the higher the fuel efficiency.The MLIT evaluates the fuel efficiency of ICE vehicles and publishes the 'List of Fuel Economy for Cars' every year based on the 'Implementation Guidelines for Evaluating and Disclosing Fuel Economy of Automobiles (MLIT Notice No. 61 of 2004)' to promote the spread of vehicles with high fuel efficiency.However, this survey now still only covers the ICE vehicles and a few hybrid vehicles, and thus based on this data and combined withempirical values of some typical vehicle models (especially EVs and FCVs), we constructed the dataset of energy efficiency for each type of vehicle as of 2019 required for the model construction (table S3).

Emission intensity
Considering the availability, accuracy, and consistency of data, the environmental loading factors of the above four pollutants by different fuel types given by the IPCC EFDB database in 2016 were used as the reference values of environmental emission intensity (table S4).There is no direct emission from EVs and FCVs considered in this research.

Scenario data
Scenarios are all designed based on the data from Energy White Paper 2022, Japan Statistical Yearbook 2023, Vehicle Fuel Consumption Surveys 2020, Automobile Transportation Survey [30][31][32][33], and official reports from several automakers.

Scenario design
This research constructed different development scenarios based on the aforementioned driving factors and constructed a total of six scenarios, as shown in figure 3.

Business as usual (BAU)
Considering the significant impact of the COVID-19 pandemic on the global economy and various aspects  of society, including Japan, and the stay-at-home policies that have resulted in a significant decrease in transportation sector data, this study designates 2019 as the base year and 2021 as the starting year for policy implementation to ensure consistency in analysis and the interpretability of results.We only considered the changes in annual passenger-km and ton-km transported under the BAU and thus extrapolation of the data from 2019 to 2050 is conducted as figure 4 based on the scenario data.The extrapolation is based on official projections conducted by MLIT and we made some adjustments by stepwise regression considering other social economic driving factors [34].By 2030, with sustained economic development, annual p-Km will experience a slight increase.However, as the trends of declining birth rates and aging intensify, there will be a continuous decline after 2030.In the field of freight transportation, there is also the risk of declining birth rates and aging, and with increasing pressure for sustainability and ongoing technological advancements, annual t-Km is expected to continue decreasing.

Fuel efficient scenario (FES)
The reduction of emissions by the transportation sector has become one of the crucial measures to address climate change, and countries worldwide have also put forth corresponding goals to improve fuel efficiency.Since the introduction of the Top Runner standard (which sets the performance standards based on the most energy-efficient products currently the market, considering factors such as prospects for technological development) in 1999 for automobiles in Japan, the fuel efficiency standards for vehicles have undergone several changes [35].The historical real data of average fuel efficiency of passenger vehicles and the standard targets by year are shown in figure 5.The standard targets for heavy-duty vehicles have also undergone several revisions.The targets for freight trucks and buses have decreased from 14.88 l/100 km and 17.79 l/100 km in 2015 to 13.11 l/100 km and 15.34 l/100 km in 2025, which shows a 13.4% and 14.3% improvement, respectively [36].Due to the consideration of more refined vehicle classifications in this study, we have further designed fuel efficiency standard targets for different vehicle types by 2035 and 2050 (tables S5 and S6).

Emission restriction scenario (ERS)
The emission standards for passenger cars in Japan have undergone multiple revisions.Currently, the main limits are set for CO (carbon monoxide) at 1.15 g km −1 , HC (hydrocarbons) at 0.1 g km −1 , and NOx (nitrogen oxides) at 0.05 g km −1 .The US and Japan tend to regulate NOx strictly, considering them harmful.In contrast, in Europe, there is a focus on and strict regulation of CO 2 emissions.Restrictions on CO 2 emissions in Japan are primarily achieved by regulating fuel efficiency and by conversion, Japan's CO 2 emission standards is 74 g km −1 for the year 2030 [37].However, the EU has set the standard to 49.5 g km −1 by 2030 and zero emission after 2035 [38].
Although Japan has set a target to ban the sale of pure ICE vehicles starting from 2035, this prohibition does not encompass non-plug-in HEVs and thus, there will still be direct emissions from vehicles.Japan has not updated its emission standards for passenger cars since 2010 and has not issued any statements regarding new regulations.Considering the policies implemented by other countries worldwide and the specific circumstances in Japan, we have set the following targets for reducing emission intensity where the emission intensity reduction is considered as a 15% and 25% reduction in CO 2 and a 10% and 20% in other gases by 2035 and 2050, respectively.

EV promotion scenario (EPS)
Japan, a major automotive manufacturing nation renowned for its automotive industry and technological advancements, is being criticized for being slow in the transition to EVs.Although the number of EVs and PHEVs sold in Japan reached another record high of 139 300 units in the year to November 2023, accounting for 4% of the total number of units sold, there is still a big gap compared to China, where sales of new energy vehicles have accounted for 40.4% of the total in November 2023 [39,40].Considering the actual situation in Japan, we have further designed two sub-scenarios.The first one is the Active Promotion Scenario (APS), which represents the Japanese government and related industries accelerating the process of transitioning to EVs and proposing more aggressive policies while rapidly improving the corresponding infrastructure.The second one is the Limited Effort Scenario (LES), where the Japanese government will continue to promote the sales and adoption of EVs in Japan.However, due to an immature industry chain and low social acceptance, the penetration of EVs will only increase slightly, still leaving a significant gap compared to other leading countries, where EV adoption is growing at a faster pace.The detailed data of these two sub-scenarios are shown in tables S7 and S8.

Optimal development scenario (Combined)
The optimal development scenario (Combined) is a unique analytical tool provided by the LEAP model, comprehensively incorporates the influencing factors from all the designed scenarios.It represents a more ideal scenario.In this scenario, the annual p-km and t-km transported remain the same as in the BAU scenario.Fuel economy is consistent with the FES, and emission standards align with the ERS.The transport mode shares for each vehicle type are the same as in the APS.This scenario aims to consider the corresponding energy consumption and pollutant emission reductions in the transportation sector under the combined effect of various policy measures.It can provide policymakers with more in-depth insights for decision-making.

Energy consumption
Under all examined scenarios, the transportation sector of Japan demonstrates a consistent declining trend in total energy consumption (figure 6).Notably, BAU exhibits the slowest decline, while the Combined scenario showcases the most rapid reduction.ERS is not taken into account as its parameters do not directly pertain to energy considerations.The base year records a total energy consumption of 2441 million GJ within the transportation sector.As the policies designed in 2021 are progressively implemented, the disparities in energy consumption patterns become increasingly apparent.Under the BAU scenario, the total energy consumption is projected to decrease to 2207 million GJ by the year 2050, with an annual reduction rate of 1.1 million GJ per year.In terms of the three sub-scenarios considered, LES displays the highest total energy consumption, trailed by the FES, while the APS exhibits the lowest consumption, which is 1890 million GJ for LES, 1770 million GJ for FES, and 1389 million GJ for APS, respectively.In the comprehensive Combined scenario, which incorporates a synthesis of all sub-scenario policies, the total energy consumption in the transportation sector reaches a notably minimal level, amounting to a mere 1068 million GJ.This achievement reflects a substantial reduction of 56% when compared to the baseline BAU scenario.Such a pronounced decrease underscores the considerable effectiveness of a multifaceted policy integration approach in curtailing the overall energy requirements within the transportation sector.
Additionally, we have also calculated the energy consumption of gasoline, diesel, electricity, and hydrogen under each scenario by 2050 (figures 7 and 8).In the base year, the consumption of gasoline and diesel was 49.4 and 23.3 billion-L, and both show declining trends under the BAU where a 5% decrease in gasoline and a 15% decrease in diesel due to declining annual transportation volumes can be observed.The reduction rate in gasoline and diesel consumption under APS relative to BAU amounted to an impressive 45% and 43%, whereas under LES, the reduction rate is 16% and respectively.Under the comprehensive Combined scenario that integrates various sub-scenarios, the consumption of gasoline and diesel experienced a further decrease to 57% compared to BAU.These findings highlight the significance of the adoption of EVs and FCVs as a key determinant in reducing fuel consumption.The comprehensive Combined scenario further magnifies the potential for reducing fuel consumption through a combination of technological advancements, policy interventions, and the adoption of EVs and FCVs in the transportation sector.As an island nation with an extremely low energy self-sufficiency rate, reducing the dependence on gasoline and diesel will also contribute to further enhancing Japan's energy security.
However, with the widespread adoption of EVs and FCVs, there are also concerns about whether the existing power grid and hydrogen technologies can support the future energy demand.It is difficult to accurately predict the extent of future EV and FCV adoption at present, but it is anticipated that electricity and hydrogen demand will increase in line with the level of their penetration rate.Japan's total electricity demand in 2019 was 877 TWh, whereas the consumption attributable to EVs at that time was very low due to their limited adoption, amounting to only 166 GWh.Under the LES, the overall electricity consumption of the transportation sector will reach 4.4 TWh by 2030, and 12.2 TWh by 2050, which is not particularly significant while under the APS, the electricity consumption of the transportation sector will reach 13.5 TWh by 2030, and 38 TWh by 2050, showing a considerably faster growth compared to the LES.Regarding hydrogen energy, the development of hydrogen FCVs is still in its early infrastructure and cost factors.Under the LES, FCVs are primarily introduced in some buses and freight trucks.By the year 2030, the projected hydrogen energy consumption in the transportation sector is estimated to reach 20 000 tons, and by 2050, 52 500 tons.The overall growth rate is relatively slow.However, under the APS, we have considered more proactive policy incentives, leading to a higher proportion of FCV adoption in private vehicles.Consequently, it is estimated to reach 77 200 tons, and 259 500 tons by 2030 and 2050.Respectively, which is nearly five times the consumption under the LES.Under the Combined Scenario, thanks to the comprehensive effects of various efforts, the total consumption of hydrogen energy is 20% lower compared to the APS stages, primarily due to limitations in refuelling.

Pollutant emission
This research considered four types of pollutants mainly emitted in the transportation sector including CO 2 , Carbon Monoxide (CO), Methane (CH 4 ), and Nitrous Oxide (N 2 O).The CO 2 emission includes the direct CO 2 emissions from fuel vehicles and the indirect carbon emissions from the fuel cycles of EVs and FCVs based on the carbon intensity of electricity and hydrogen production which are listed in tables S8 and S9 [41]. Figure 9 shows the model results.The CO 2 emission in the base year was 172.7 million tons and is expected to decrease to 156 million tons by 2050.The ERS enforces stricter emission standards for vehicles and is projected to have 20% emissions cut of 125 million tons by 2050, which shows a similar reduction rate to FES.Under the two sub-scenarios of EPS, LES represents a more conservative policy approach, which contributes less to emission reductions compared with ERS and FES, with estimated emissions of 130 million tons by 2050, only a 16% reduction compared with BAU.In contrast, under the APS, the Japanese government proactively promotes the adoption of electric vehicles.
As a result, emissions are expected to decrease to 86 million tons by 2050, a significant 45% reduction compared with BAU.Furthermore, under the Combined scenario, a more comprehensive and integrated approach leads to a notable decrease in emissions, with a total of 67% reduction to 52 million tons of CO 2 emission by 2050.The emissions of the other three pollutants likewise exhibited comparable tendencies with a total emission of CO, CH 4, and N 2 O of 21 million tons, 45 700 tons, and 16 300 tons in the base year, respectively.By 2050, the emissions are projected to drop to 6.8 million tons, 13 800 tons, and 5200 tons under the Combined scenario, which is about 66% cut compared to BAU.

Discussion and conclusion
Japan has set goals for reducing 46% GHGs emission by 2030 (compared to 2013) and achieving carbon neutrality by 2050.To attain these objectives, the transportation sector, one of the largest contributors to Japan's carbon emissions, needs to reduce its emission to 50 million tons by 2050 [42] by enhancing the fuel efficiency of vehicles, reducing emission intensity, and accelerating the transition to EVs and FCVs, which are in line with the three sub-scenarios designed in this research, together with a BAU scenario as baseline and a Combined scenario as the optimal development pathway.
Results show that total energy consumption consistently decreases under all scenarios, with the Combined scenario showing the most significant reduction of 56% compared to BAU.The consumption of gasoline and diesel has decreased by 16% to 18% under LES and FES, and it has reached 45% under APS.This will significantly reduce Japan's dependence on fossil fuels and enhance energy security.Despite there is a substantial increase in the demand for electricity and hydrogen to 30 ∼ 40 TWh and 200 ∼ 250 thousand Tons by 2050, however, this only accounts for 1.5% of the planned electricity supply and 0.1% of the hydrogen supply by 2030.Even by 2050, the proportions are around 3% and 1%, respectively, implying that it will not impose a significant burden on the energy supply system [41,43].Besides, pollutant emissions are significantly reduced in all scenarios, but only the Combined scenario can achieve the target of 50 million tons of CO 2 emissions by 2050.
Compared to other scenario studies on the transportation sector [44][45][46], there is currently very few systematic scenario analysis research focused on the transportation sector in Japan.Besides, this research not only considers a broader classification of vehicle types with different energy sources but also designs two scenarios of different EV adoption rate to analyze the gap between slow and fast transitions in Japan.Additionally, a combined scenario is devised to examine the impact of the compounded effects of multiple policies on the energy transition in the transportation sector.It Is undeniable that the current electricity and hydrogen production still heavily rely on fossil fuels, leading to significant lifecycle carbon emissions from EVs and FCVs.However, with Japan's introduction of the sixth energy strategy and a new basic hydrogen strategy [47,48], existing research has confirmed a substantial reduction in the carbon intensity of electricity generation under Japan's nationally determined contributions [41].This study, taking into account this evolving context, further validates the positive significance of EV adoption for the sustainable development in the transportation sector.
In summary, the scenario analysis of Japan's transportation sector presents a clear trajectory of the energy transition in Japan's transportation sector and showed that only the combination of a comprehensive policy approach can substantially cut the energy consumption and pollutant emissions to achieve the 2050 carbon neutral goal, and the adoption of EVs and FCVs play a key role in the pocess.Building upon these findings, it is recommended to enforce more stringent fuel efficiency and emission standards in the short term, while accelerate infrastructure development, and incentivizing EVs and FCVs through measures such as tax reductions and subsidies to further increase market share to facilitate a comprehensive energy transition in the transportation sector.This study can provide valuable insights for Japan's roadmap to carbon neutrality by 2050.

Figure 5 .
Figure 5. Trends in the average fuel efficiency and standard targets of gasoline passenger vehicles.

Figure 6 .
Figure 6.Energy consumption of Japan's transportation sector under different scenarios.