Logarithmic Mean Divisia Index Decomposition of CO2 Emissions from Urban Passenger Transport: An Empirical Study of Global Cities from 1960-2001

: The urban transport sector has become one of the major contributors to global CO 2 emissions. This paper investigates the driving forces of changes in CO 2 emissions from the passenger transport sectors in di ﬀ erent cities, which is helpful for formulating e ﬀ ective carbon-reduction policies and strategies. The logarithmic mean Divisia index (LMDI) method is used to decompose the CO 2 emissions changes into ﬁve driving determinants: Urbanization level, motorization level, mode structure, energy intensity, and energy mix. First, the urban transport CO 2 emissions between 1960 and 2001 from 46 global cities are calculated. Then, the multiplicative decomposition results for megacities (London, New York, Paris, and Tokyo) are compared with those of other cities. Moreover, additive decomposition analyses of the 4 megacities are conducted to explore the driving forces of changes in CO 2 emissions from the passenger transport sectors in these megacities between 1960 and 2001. Based on the decomposition results, some e ﬀ ective carbon-reduction strategies can be formulated for developing cities experiencing rapid urbanization and motorization. The main suggestions are as follows: (i) Rational land use, such as transit-oriented development, is a feasible way to control the trip distance per capita; (ii) fuel economy policies and standards formulated when there are oil crisis are e ﬀ ective ways to suppress the increase of CO 2 emissions, and these changes should not be abandoned when oil prices fall; and (iii) cities with high population densities should focus on the development of public and non-motorized transport.


Introduction
Climate change and CO 2 emissions mitigation have drawn extensive attention worldwide in recent years. Because of its continuously growing share in overall energy consumption, the transport sector has been acknowledged as one of the most important contributors to global emissions [1]. According to the International Energy Agency, world energy-related CO 2 emissions will increase from 32.3 billion metric tons in 2012 to 35.6 billion metric tons in 2020 and to 43.2 billion metric tons in 2040 [2]. Besides, 7.38 million tons of CO 2 was generated due to oil consumption in the global transport sector in 2013, accounting for 23% of the total fossil fuel-related CO 2 emissions [3]. In addition, with the continuous development of the urban economy and acceleration of the motorization process, cities account for 75% of global energy consumption and 80% of greenhouse gas emissions [4]. Therefore, the

Literature Review
Index decomposition analysis (IDA) method is one of the main approaches for investigating different driving factors and their environmental side effects in a harmonized way [6]. The basic concept of the IDA method is to decompose one target variable into a combination of many factors and then determine how much each factor affects the result, namely, the contribution. The main IDA method can be further categorized into the Divisia index method and the Laspeyres index method. For the Laspeyres index method, there are always residual items that cannot be merged and ignored in the process of decomposition, which has side effects on the result of decomposition. For the Divisia method, there are no residual terms, thus it has gradually become the mainstream method of empirical research in the academic field. Furthermore, logarithmic mean Divisia index (LMDI) is a typical Divisia index method that is convincing both in theory and application. Ang and Liu introduced a refined Divisia index method, the LMDI approach, which is characterized by perfect decomposition and consistency in aggregation [7]. Then, Ang compared various IDA methods and concluded that the LMDI method was the preferred method [8].
In the transport sector, several studies have been conducted to examine factors that have affected changes in energy consumption and emissions over the past few decades. From national and regional perspectives, Schipper et al. compared the CO 2 emissions growth from passenger transport and freight transport in some countries from the Organization for Economic Cooperation and Development between 1973 and 1992 [9,10]. Moutinho et al. identified the relevant factors that have influenced the changes in the level of CO 2 emissions among four groups of European countries, specifically eastern, western, northern, and southern Europe groups, based on the LMDI approach from 1999 to 2010 [11]. Yeo et al. used the LMDI decomposition method to identify and analyze the key driving forces behind changes in CO 2 emissions in two emerging countries: China and India [12]. Timilsina and Shrestha performed an LMDI analysis of CO 2 emissions in the overall transport sector in different Asian countries and identified different driving forces of CO 2 emissions in these countries [13]. Li et al. studied CO 2 emissions performance at both the national and regional levels; they traced the growth trend and spatial disparity of CO 2 emissions in China based on the LMDI method from 2000-2014 [14]. Using the LMDI method, Zhang et al. identified the relationships between transport sector energy consumption and changes of the transport mode, passenger-freight share, energy intensity, and transport activity in China between 1980 and 2006 [15]. Jiang et al. presented the CO 2 emissions trends from the transport sector at the Chinese provincial level and then quantified the related driving forces by adopting LMDI analysis [16].
From the city-scale perspective, Wang and Hayashi adopted the LMDI technique to decompose the total passenger transport CO 2 growth in Shanghai from 2000 to 2009 into five driving factors: Economic activity, population, modal share, passenger transport intensity, and passenger transport CO 2 emissions factor [17]. Then, this method was used to compare the CO 2 emissions from the urban Sustainability 2019, 11, 4310 3 of 16 passenger transport sectors in Shanghai and Tokyo. The driving factors of decomposition analysis were determined to be the population, trip generation rate, mode shift, travel distance, and load effect [18].
Most of these studies focused on national-level transportation systems, and only a few conducted city-scale transport sector decomposition analyses. However, to the best of our knowledge, the trends and driving forces of CO 2 emissions from the passenger transport sector in various cities, especially megacities with large populations and total emissions, have not been explicitly studied. However, as a result of imbalanced development worldwide, different cities are facing different challenges, leading to different CO 2 emissions trends. To fill this gap, this paper performs a comparative study of 46 cities to identify the driving forces of CO 2 emissions from the passenger transport sector between 1960 and 2001 by dividing these cities into megacities and other cities. Further analysis is conducted for the megacities, and the main driving forces of the CO 2 emissions from urban passenger transport in these megacities are identified to provide experience for other cities.

Research Area
This paper studies CO 2 emissions from the passenger transport systems (excluding aviation and ferry transport) of 46 cities in 1960, 1970, 1980, 1990, 1996, and 2001. According to the research of the International Association of Public Transport [19], these cities can be categorized into four regions considering their geographic locations as shown in Figure 1 Most of these studies focused on national-level transportation systems, and only a few conducted city-scale transport sector decomposition analyses. However, to the best of our knowledge, the trends and driving forces of CO2 emissions from the passenger transport sector in various cities, especially megacities with large populations and total emissions, have not been explicitly studied. However, as a result of imbalanced development worldwide, different cities are facing different challenges, leading to different CO2 emissions trends. To fill this gap, this paper performs a comparative study of 46 cities to identify the driving forces of CO2 emissions from the passenger transport sector between 1960 and 2001 by dividing these cities into megacities and other cities. Further analysis is conducted for the megacities, and the main driving forces of the CO2 emissions from urban passenger transport in these megacities are identified to provide experience for other cities.

Research Area
This paper studies CO2 emissions from the passenger transport systems (excluding aviation and ferry transport) of 46 cities in 1960, 1970, 1980, 1990, 1996, and 2001. According to the research of the International Association of Public Transport [19], these cities can be categorized into four regions considering their geographic locations as shown in Figure 1, including North American cities, Oceanian cities, European cities, and Asian cities. The North American cities include Boston, Chicago, Denver, Detroit, Houston, Los Angeles, New York, Phoenix, Portland, Sacramento, San Diego, San Francisco, Washington, Toronto, Calgary, Winnipeg, Edmonton, Montreal, Ottawa, and Vancouver. The Oceanian cities include Adelaide, Brisbane, Canberra, Melbourne, Perth, and Sydney. The European cities include Amsterdam, Copenhagen, Frankfurt, Hamburg, London, Munich, Paris, Stockholm, Vienna, Zurich, and Brussels. The Asian cities include Hong Kong, Tokyo, Singapore, Bangkok, Djakarta, Kuala Lumpur, Manila, Seoul, and Surabaya. In this study, London, Paris, New York, and Tokyo are defined as megacities according to the population and metropolitan gross domestic product per capita.

Data
The data include annual vehicle kilometers traveled, passenger kilometers traveled, fuel consumption data for all passenger transport modes, and population data for 46 cities in 1960, 1970, 1980, 1990, 1996, and 2001. Data used in this study come from An International Sourcebook of Automobile Dependence in Cities, 1960Cities, -1990 and the Millennium Cities Database for Sustainable Transport [20].

Passenger Transport CO 2 Emissions Calculation
The CO 2 emissions from each city's urban transport sector were calculated based on the accounting method described in Guidelines for National Greenhouse Gas Inventories [21], as shown in Equation (1): where CO 2 represents the total energy consumption-related CO 2 emissions from the passenger transport sector in a city, j denotes the type of energy source, EC j denotes the energy consumption of fuel j, and EFj denotes the CO 2 emissions factor of fuel j. The CO 2 emissions factors of various kinds of fuels from the Guidelines for National Greenhouse Gas Inventories are used in this study. Because this study emphasizes the direct CO 2 emissions from the transport sector derived from end-use energy consumption, indirect CO 2 emissions from the transport sector, such as CO 2 emissions related to electricity consumption and fuel production, are not included.

Decomposition Methodology
The LMDI analysis of CO 2 emissions from the passenger transport sector in each city is conducted based on Equation (2).
Equation (2) can be shortened to where C represents the total energy consumption-related CO 2 emissions from the passenger transport sector in one city, C ij is the CO 2 emissions from passenger transport mode i with energy type j, P is the population of the city, L i is the annual passenger kilometers traveled via mode i, L is the total annual passenger kilometers traveled by all transport modes, E i is the energy consumption by passenger transport mode i, E ij is the energy consumption from passenger transport mode i with energy type j, and l is the annual passenger kilometers traveled per capita. m i refers to the share of travel of mode i in terms of passenger kilometers. em ij is the energy share of type j in mode i. f j is the CO 2 emissions factor of energy type j. i = 1 represents private transportation, and i = 2 represents public transportation. j = 1 represents gasoline, j = 2 represents natural gas, j = 3 represents diesel, and j = 4 represents electricity.
In additive decomposition, the effects of various driving factors from the baseline year 0 to the final year t can be expressed as follows.
The various driving forces can be quantified according to the following equations.
In multiplicative decomposition, the effects of various driving factors from the baseline year 0 to the final year t can be expressed as follows.
The various driving forces can be quantified according to the following equations.
Some key factors play significantly important roles in the change of carbon emissions in urban transport. Lee Schipper established the ASIF framework model for carbon emissions from transport sector, which represents activity, structure, intensity, and fuels [22]. On this basis, this study further divided the ASIF framework model into five factors: Namely, "urbanization effect" (∆Cp, Dp), "motorization effect" (∆Cl, Dl), "mode structure effect" (∆Cm, Dm), "energy intensity effect" (∆Cei, Dei), and "energy mix effect" (∆Cem, Dem). Sustainability 2019, 11, 4310 6 of 16 "Urbanization effect" (∆Cp, Dp) is generally measured by urban population size. "Motorization effect" (∆Cl, Dl) is measured by passenger kilometers per capita. "Mode structure effect" (∆Cm, Dm) has a great impact on the carbon emissions from urban transport. Different travel modes have different carbon emission intensities. They are ranked from high to low: Single-passenger cars, high-capacity cars, taxis, commerce vehicles, public transportation, bicycles, and walking [23]. "Energy intensity effect" (∆Cei, Dei) mainly refers to the energy consumption per unit kilometer. "Energy mix effect" (∆Cem, Dem) is a direct factor in determining the level of carbon emissions from urban transport. Different kinds of fuels have different carbon emission factors. It can be found that traditional fossil fuels such as gasoline and diesel have higher emission factors. Electric energy and hydrogen fuel produce no carbon emissions during the operating phase [24].
Large values of ∆C and D reflect large contributions of the driving factor. In this study, we use the emission factors from the Guidelines for National Greenhouse Gas Inventories [21] and assume that the emissions factors of various energy sources remained unchanged during the study period. Thus, f it f i0 = 1, and the emissions factor effect is 0.

CO 2 Emissions Calculation Results
CO 2 emissions from the urban passenger transport sector in 46 cities were calculated with the method presented above. The results are shown in Table 1. Figure 2 shows the CO 2 emissions growth rate from 1960 to 2001. As shown in Figure 2, urban passenger transport CO 2 emissions from both megacities and other cities experienced decelerated growth over the study period. Additionally, CO 2 emissions from the urban passenger transport in both mega cities and other cities experienced a reduction from 1996 to 2001. Specifically, urban passenger transport CO 2 emissions peaked during the 1990s for most cities over the study period. High CO 2 emissions cities are mainly distributed in the United States, such as New York, Los Angeles. European cities such as Copenhagen and Zurich have relatively low emissions.  Figure 2 shows the CO2 emissions growth rate from 1960 to 2001. As shown in Figure 2, urban passenger transport CO2 emissions from both megacities and other cities experienced decelerated growth over the study period. Additionally, CO2 emissions from the urban passenger transport in both mega cities and other cities experienced a reduction from 1996 to 2001. Specifically, urban passenger transport CO2 emissions peaked during the 1990s for most cities over the study period. High CO2 emissions cities are mainly distributed in the United States, such as New York, Los Angeles. European cities such as Copenhagen and Zurich have relatively low emissions.

CO2 Emissions Decomposition Results-A Comparison of Megacities and Other Cities
The LMDI method is used to decompose the CO2 emissions trends from the urban passenger transport sector into five driving factors: The urbanization effect (Dp), motorization effect (Dl), mode structure effect (Dm), energy intensity effect (Dei), and energy mix effect (Dem). The average values of the multiplicative decomposition results for megacities (London, Paris, New York, and Tokyo) are calculated and compared with those of other cities (see Figure 3a-e).

CO 2 Emissions Decomposition Results-A Comparison of Megacities and Other Cities
The LMDI method is used to decompose the CO 2 emissions trends from the urban passenger transport sector into five driving factors: The urbanization effect (Dp), motorization effect (Dl), mode structure effect (Dm), energy intensity effect (Dei), and energy mix effect (Dem). The average values of the multiplicative decomposition results for megacities (London, Paris, New York, and Tokyo) are calculated and compared with those of other cities (see Figure 3a-e). The LMDI method is used to decompose the CO2 emissions trends from the urban passenger transport sector into five driving factors: The urbanization effect (Dp), motorization effect (Dl), mode structure effect (Dm), energy intensity effect (Dei), and energy mix effect (Dem). The average values of the multiplicative decomposition results for megacities (London, Paris, New York, and Tokyo) are calculated and compared with those of other cities (see Figure 3a-e). From 1960 to 1970, five driving forces had positive effects on passenger transport sector CO2 emissions growth. For megacities, the urbanization effect (Dp) and motorization effect (Dl) were the most dominant driving factors that increased CO2 emissions. The contributions of these two factors to passenger transport sector CO2 emissions growth in other cities were larger than those in megacities because the urbanization and motorization processes in other cities were more rapid from 1960-1970. Modal shifting to private transport modes also substantially contributed to the passenger transport CO2 emissions growth in megacities, but this impact was small in other cities (see Figure  3a). From 1960 to 1970, five driving forces had positive effects on passenger transport sector CO 2 emissions growth. For megacities, the urbanization effect (Dp) and motorization effect (Dl) were the most dominant driving factors that increased CO 2 emissions. The contributions of these two factors to passenger transport sector CO 2 emissions growth in other cities were larger than those in megacities because the urbanization and motorization processes in other cities were more rapid from 1960-1970. Modal shifting to private transport modes also substantially contributed to the passenger transport CO 2 emissions growth in megacities, but this impact was small in other cities (see Figure 3a).
From 1970 to 1980, urban transport CO 2 emissions in both types of cities increased by a factor of approximately 1.4 on average compared to the previous value as shown in Figure 2. The main contributor to the growth was the increase in trip distance per capita (motorization effect, Dl). The energy intensity effect (Dei) was a positive driving factor of the passenger transport CO 2 increase of megacities but a negative factor in other cities (see Figure 3b).
From 1980 to 1990, each driving factor played a similar role in megacities and other cities. Although an overall increasing trend was observed, the energy intensity effect (Dei) and energy mix effect (Dem) partially offset the increasing effects contributed by urbanization effect (Dp), motorization effect (Dl), and mode structure effect (Dm) (see Figure 3c).
From 1990 to 1996, the urban passenger transport CO 2 emissions in megacities displayed a modest rise, which was mainly caused by modal shifting to personal transport modes (mode structure effect, Dm). Additionally, the energy intensity effect (Dei) offset some increasing effects on CO 2 emissions in megacities and other cities (see Figure 3d).
From 1996 to 2001, urban passenger transport CO 2 emissions from both megacities and other cities significantly declined. Motorization effect (Dl) and mode structure effect (Dm) were the main driving forces of the reduction in passenger transport CO 2 emissions (see Figure 3e).

CO 2 Emissions Decomposition Results for the Four Megacities
To investigate the driving forces of changes in CO 2 emissions from the passenger transport sectors in megacities, a period-series LMDI additive decomposition analysis was conducted for the period of 1960 through 2001. From 1990 to 1996, the urban passenger transport CO2 emissions in megacities displayed a modest rise, which was mainly caused by modal shifting to personal transport modes (mode structure effect, Dm). Additionally, the energy intensity effect (Dei) offset some increasing effects on CO2 emissions in megacities and other cities (see Figure 3d).
From 1996 to 2001, urban passenger transport CO2 emissions from both megacities and other cities significantly declined. Motorization effect (Dl) and mode structure effect (Dm) were the main driving forces of the reduction in passenger transport CO2 emissions (see Figure 3e).

CO2 Emissions Decomposition Results for the Four Megacities
To investigate the driving forces of changes in CO2 emissions from the passenger transport sectors in megacities, a period-series LMDI additive decomposition analysis was conducted for the period of 1960 through 2001.               Decomposition Results for London. As Figure 4 indicates, generally the motorization effect (∆Cl) and mode structure effect (∆Cm) are sensitive factors related to CO 2 emissions change. Specifically, the effect of motorization (∆Cl) is positive from 1960 to 1990 and negative from 1990 to 2001. The mode structure effect (∆Cm) appears to be positive for the periods of 1960-1980 and 1990-1996 and negative for the periods of 1980-1990and 1996. From 1960to 1980, motorization effect (∆Cl) and mode structure effect (∆Cm) were the main contributors to the CO 2 emissions increase from the passenger transport sector in London. Motorization effect (∆Cl) contributed to 49.5% and 78.3% of the total change for the period of 1960-1970 and 1970-1980, respectively. Mode structure effect (∆Cm) contributed to 57.6% and 62.0% of the total change for the periods of 1960 to 1970 and 1970 to 1980, respectively. During the period of 1960-1990, the urbanized area in London expanded. Therefore, the motorization effect (∆Cl) reached a historical peak as a result of urban sprawl. Additionally, counter-urbanization led to a scattered population, a longer average trip distance and an increased private transport share, thereby promoting carbon emissions growth. From 1980 to 1990, the inhibitory effects of the mode structure (∆Cm) and energy intensity (∆Cei) weakened the growth trend of CO 2 emissions. The three oil crises that occurred between 1973 and 1990 made the British government focus more on the development of public transport than in the past, and technological progress further reduced the energy intensity.
Decomposition Results for New York. As Figure 5 indicates, the total CO 2 emission from the urban passenger transport sector in New York increased from 1960 to 1990 and then decreased from 1990-1996. Overall, the largest contributor was the motorization effect (∆Cl), followed by the mode structure effect (∆Cm). The energy intensity effect (∆Cei) constantly negatively contributed to CO 2 emissions, and the contributions in the periods of 1960-1970, 1970-1980, 1980-1990, and 1990-1996 were −9.4%, −205.8%, −266.5%, and −411.2%, respectively. The impact of this factor increased over time because the Corporate Average Fuel Economy (CAFE) standards were enacted in 1975 after the first oil crisis. The fuel economy of vehicles in 1985 was twice as high as that in 1975, and the energy intensity effect (∆Cei) has become the key negative factor to offset the increase of CO 2 emissions since the 1980s. During the same time period, the average trip distance per capita increased as a result of counter-urbanization; thus, the motorization effect (∆Cl) promoted CO 2 emissions growth.
Decomposition Results for Paris. As Figure 6 illustrates, CO 2 emissions from urban passenger transport in Paris increased from 1960 to 1996 and then decreased from 1996-2001. From 1960 to 1996, motorization effect (∆Cl) and mode structure effect (∆Cm) were the key contributors to the urban passenger transport CO 2 emissions increase in Paris. In 1965, Paris proposed the plan to construct new cities with low population densities in the surrounding area. Additionally, the travel distance and private transport share increased, which contributed to the increase in CO 2 emissions. However, the Paris government approved the "Seine Rive Gauche" zone development plan in 1991. The local government has changed this area from an industrial land into a livable area by integrating housing, offices, services, and general amenities [25]. The mix of everyday destinations, rather than individual isolated space, makes travel distances shorter and makes it easier to improve public transportation. The average travel distance became shorter and the public transport share began to rise due to the operation of metro line 14 in 1998. Therefore, the motorization effect (∆Cl) and mode structure effect (∆Cm) were the key contributors to the urban passenger transport CO 2 emissions decrease in Paris in the period of 1996-2001.
Decomposition Results for Tokyo. As Figure 7 shows, the CO 2 emissions from urban passenger transport in Tokyo steadily increased from 1960 to 1996. Due to rapid urbanization and the popularization of private cars in Tokyo, urbanization effect (Dp), motorization effect (Dl), and mode structure effect (Dm) promoted the growth of CO 2 emissions. The "Energy Conservation Act", which focused on improving the fuel economy, was implemented in 1979. Consequently, the energy intensity effect (∆Cei) became the most important negative factor related to the growth of CO 2 emissions. Specifically, the energy intensity effect (∆Cei) has reduced 250,350 tons of CO 2 emissions in the period of 1980-1990.

Policy Implications
After evaluating the contributions of five key driving forces of CO 2 emissions from urban transport, some policy implications involved with these five factors are recommended to develop low-carbon urban transport planning.
For the "urbanization effect", controlling the urban population size is a feasible way to decrease carbon emissions from urban transport. The global population will continue to grow over the next few decades. Even though the population of some developed countries has stabilized or even declined, the population of developing countries has exploded. It is estimated that by 2100, the total global population will exceed 10 billion [26]. The Chinese government clearly states the aim of "strictly controlling the population size of megacities" in the National New Urbanization Plan (2014-2020) [27].
For the "motorization effect", it has contributed more to the CO 2 emissions in the United States cities than in other mega cities according to the above decomposition result. Millard-Ball and Schipper also revealed that the average motorized travel distance per capita in the United States was twice that in Japan under the same level of the per capita GDP [28].
For the "mode structure effect", private cars have a higher energy intensity than public and non-motorized transport. Bristow A. L. et al. suggested that adjusting the mode structure was the most effective way to decrease the CO 2 emissions from urban transport [29]. Most North American cities are car-oriented. The number of global car ownership will rapidly increase to 2 billion in the next few decades, most of which are from developing countries [30]. Therefore, it is really important for the government to advocate and improve public or non-motorized transport services.
For the "energy intensity effect", many cities took measures to improve the fuel efficiency in the 1980s because of oil crisis. Consequently, the "energy intensity effect" was the key negative factor to offset the increase of CO 2 emissions from urban transport in the 1980s. The increased fuel economy also reduces the cost of using vehicles, thus leading to the increase of the motorization, which is called the "rebound effect" [31]. Mishina and Muromachi identified an increase in fuel prices by implementing a fuel tax increase, which would be one feasible method to improve the on-road fuel economy and reduce the rebound effects [32]. Therefore, fuel economy policies and standards formulated when oil prices are high should not be abandoned when oil prices fall. Furthermore, high-capacity cars can also decrease the fuel use per capita. The shared mobility service can be an effective way to decrease the CO 2 emissions from urban transport.
For the "energy mix effect", electric energy and hydrogen fuel have lower emission factors than traditional fossil fuels such as gasoline and diesel during the operating phase. The interest in electric-powered vehicles has rekindled worldwide in recent years because of global climate change. It is estimated that the number of electric cars would need to exceed 700 million by 2040 [2]. The transportation fuels will show a diversified trend in the future, including not only traditional fossil fuels, but also alternative energy such as electric energy, biofuels, and clean natural gas.
As shown in Table 2, this paper sorts out the typical urban low-carbon transport policy rules, which can be divided into five categories based on the involved target factors. Furthermore, these rules can be subdivided into 14 types of measures: Urbanization measures mainly include controlling the urban population; motorization measures include constructing facilities suitable for non-motorized transport, increasing land mix and promoting telecommuting; mode structure measures include constructing public transport facilities, controlling the number of license plates, tax collection, congestion charge, and parking management; energy intensity measures include fuel economy requirements and management of scrap cars; energy mix measures include constructing charging stations for electric vehicles, subsidizing vehicles with low-emission and advocating shared electric vehicles.

Conclusions and Suggestions
This paper presented a holistic picture of CO 2 emissions from the urban passenger transport sectors in 46 cities. By conducting a period-wise LMDI analysis, this study quantified how the population, trip distance per capita, mode structure, modal share, energy intensity, and energy mix contributed to CO 2 emissions changes in the urban passenger transport sector during the period of 1960-2001 in different cities. The contributions of different driving factors in megacities and other cities were compared. Furthermore, detailed analyses of carbon emissions from urban passenger transport in the four megacities were conducted to identify the main driving factors. Finally, this paper classified all kinds of policy rules based on the involved target factors to provide practical guidance to low-carbon urban planning in developing countries. The main research outcomes are as follows: (i) Urban passenger transport CO 2 emissions in both megacities and other cities experienced decelerated growth and a reduction from 1960-2001. It peaked during the 1990s for most cities over the study period. High CO 2 emissions cities are mainly distributed in the United States, such as New York, Los Angeles. European cities such as Copenhagen and Zurich have relatively low emissions.
(ii) From 1960 to 1970, the contributions of the urbanization effect (Dp) and motorization effect (Dl) to passenger transport sector CO 2 emissions growth in other cities were larger than those in megacities because urbanization and motorization processes in other cities were more rapid from [1960][1961][1962][1963][1964][1965][1966][1967][1968][1969][1970]. From 1960 to 1996, mode structure effect played a more important role in influencing CO 2 emissions from the urban passenger transport in megacities than in other cities. From 1996 to 2001, the main inhibitory effects of CO 2 emissions growth were mainly from the improvement of the public transport share and reduction in the trip distance per capita in both types of cities. (iii) Energy intensity effect was the main inhibitory factor of urban passenger CO 2 emissions growth in megacities (London, Paris, New York, and Tokyo) from 1980-1990 because the government focused on improving the fuel economy of motor vehicles after the oil crisis. From 1970 to 1990, counter-urbanization led to longer trip distances and increased private transport shares, thereby promoting CO 2 emissions growth in megacities.
Based on this investigation of the driving forces of changes in CO 2 emissions from the passenger transport sectors in 46 cities, including 4 megacities, some effective carbon-reduction policies and strategies can be formulated for developing cities experiencing rapid urbanization and motorization. The main suggestions are as follows: (i) Rational land use, such as transit-oriented development, is a feasible way to make average travel distances shorter and decrease the motorization level. (ii) Fuel economy policies and standards formulated when oil prices are high are effective ways to suppress the increase of CO 2 emissions from urban transport. These policies should not be abandoned when oil prices fall. Oil crises provide important opportunities for improving the fuel economy. (iii) Cities with high population densities should focus on the development of public and non-motorized transport. Some rules should be implemented to prevent the unlimited spread of cities and counter-urbanization.
This study can provide practical guidance to low-carbon urban planning in developing countries. However, there are some limitations to the current study. Some other important cities such as Beijing and Mexico City, are not mentioned in this study because of the limit of data. This paper only chooses 4 mega cities for further analyses. More cities can be chosen according to the region or the development stage in the future. The authors recommend that future studies focus on these issues.