Influence of Demographics on Consumer Preferences for Alternative Fuel Vehicles: A Review of Choice Modelling Studies and a Study in Portugal

The significant energy consumed by road transportation and the difficult market penetration of Alternative Fuel Vehicles (AFVs) has led to a substantial body of research aiming to understand consumer preferences and future demand for AFVs. The individual characteristics of consumers are one of the explanatory factors of these preferences. In this context, the main purpose of this work is to present a comprehensive state-of-the-art review of how consumer demographics influence their preferences concerning AFVs. This review focuses on papers that applied Choice Modelling techniques to elicit individual consumer preferences for AFVs through stated preference surveys. Age, gender, income, level of education, family size, driving habits and number of vehicles per household were selected for analysis. This study also adds to the literature by analyzing the influence of demographic characteristics on preferences of Portuguese consumers. Very few studies addressed the influence of demographics on preferences for vehicle attributes. Considering the influence of consumers’ income and age, no consistent results were found. However, when age and consumers’ nationality were crossed, a potential trend of consumers’ age influence was unveiled. Regarding gender, level of education and family size, it was observed that consumers with higher education levels, women and consumers with larger families have higher preferences for AFVs.


Introduction
Road transportation is the largest energy consumer in the transport sector, e.g., representing 82% of energy consumed in Europe [1].Since most vehicles on the road use fossil fuels, this entails environmentally harmful emissions contributing to climate change and other undesirable externalities.As the impact of the transport sector on global climate change is expected to significantly worsen in the short-medium term, a rapid energy use transition is demanded in this sector [2].The aim of mitigating the environmental burden from transportation led to the development of several plans.For instance, the EU defined the Climate and Energy Package 2020 where specific targets for the transports sector mandated that, in 2020, 10% of the energy used in this sector would be from renewable sources.Alternative Fuel Vehicles (AFVs) can contribute to overcoming the environmental problem through the gradual substitution of fossil fuels by potentially more environmentally sustainable energy carriers, such as electricity, hydrogen or ethanol [3,4].However, the adoption of new technologies in the transports field worldwide has been hindered by technical and investment related concerns [5][6][7], despite large investments made by governments to increase the diffusion of AFVs, such as building

Previous Review Studies
During the last three decades, an extensive body of research has aimed at understanding consumer preferences and future demand for AFVs.Potoglou and Kanaroglou [30] did an overview of the methodological aspects of choice-based models for vehicle demand.They reviewed data collection methods (stated and revealed preferences), modelling approaches (vehicle-type choice models, vehicle-holding models and vehicle-transaction models) and also some explanatory factors of vehicle demand, such as place of residence.Al-alawi and Bradley [31] focused their study on Battery Electric Vehicles (BEVs), Hybrid Electric Vehicles (HEVs) and Plug-In Hybrid Electric Vehicles (PHEVs) market penetration literature by reviewing the most commonly used modelling techniques for market forecasting, namely agent-based models, consumer choice models, diffusion rate and time series models.A thorough review of attitudinal, experimental and preference studies is given by Turcksin et al. [27] in order to understand what the consumer attitudes and preferences for AFVs are.These authors presented the attributes, scope (type of focused vehicles), methodology applied, main findings and study location of consumer preference studies.Despite not being a review article, Hoen and Koetse [18] provide a relevant review of the main aspects of survey designs of conjoint preference studies about AFVs (type of questions, vehicles compared and selected attributes) that is useful to identify trends in these types of studies.A comprehensive overview of supportive factors and barriers to consumer adoption of AFVs was presented by Rezvani et al. [32], who reviewed the factors (attitudinal, innovative, symbolic, emotional and pro-environmental factors) that influence consumer behavior towards or against the adoption of Electric Vehicles (EVs) (in this article we use EVs as a generic class encompassing BEVs, HEVs and PHEVs).More recently, Liao et al. [33] reviewed consumer preference studies for EVs in order to suggest stronger policies to promote EVs.This review covered several topics such as the techniques used to model preferences for EVs, the preferences for attributes of EVs and the factors that account for heterogeneous preferences, namely socio-economic and demographic characteristics, psychological factors, EVs experience and social influence.
In summary, previous reviews have focused on consumer adoption and preferences for AFVs, from methodological trends in modelling preferences to explanatory factors of those preferences.However, to the authors' best knowledge, a review of the influence of individual characteristics of consumers on their preferences for AFVs has not been addressed in depth, while a review of the influence of these characteristics on preferences for vehicle attributes was never performed.Therefore, the review presented in this paper aims at filling these gaps.

Data Collection Strategy
As stated in the introduction, the main objective of this paper is to present a comprehensive state-of-the-art review of how consumer demographics influence their preferences concerning AFVs and their attributes.As the literature on consumer preferences for alternative vehicle technologies is very extensive, the review focuses on choice modelling methodologies, which are the most used group of methods to elicit preferences in the field.The reviewed studies consider mainly stated preference data, which is usual when inquiring about innovative products with scarce sales data.
Only studies published in peer-reviewed journals were considered.The study selection was done in two phases (Figure 1).The first phase comprised a Scopus®database search in the studies' title, abstract and keywords (last access in October 2018).The following keywords combination was used: "vehicle" with "preference" or "demand" and either "choice" or "conjoint."The search was limited to studies published in journals and in English.No date restriction was defined, and the selected Scopus subject areas were: Business, management and accounting; Computer science; Decision sciences; Economics, econometrics and finance; Energy; Engineering; Environmental science; Psychology; and Social sciences.The resulting output were 819 studies, which were reduced to 140 studies after the analysis of their titles.This substantial exclusion of studies was due to the multidisciplinary scope of this review.By including journals from diversified subjects, the initial list included studies that were clearly unrelated to this review, such as studies in automotive engineering or studies about choice of transportation mode.The next screening was done through the analysis of the abstracts followed by the full text reading of the remaining 86 studies.As a result, an initial selection of 60 studies was obtained.The second phase involved an analysis of the references cited by these studies in order to cover all the relevant papers, which resulted in the final selection of 79 studies that are analyzed in the next sections.

CM Studies
Understanding consumer preferences has become more complex because consumers have a wide range of choices, such as ICEVs vehicles (gasoline and diesel), BEVs, HEVs, PHEVs, Fuel Cell Vehicles (FCVs), Compressed Natural Gas Vehicles (CNGs), Natural Gas Vehicles (NGVs), and Biofuel vehicles.Therefore, consumers are constantly confronted with huge amounts of information about these vehicles, which is used by them to form preferences and, consequently, to make purchase decisions [34].This context has been justifying the interest of a substantial body of researchers that developed studies aiming at understanding consumer preferences concerning these vehicles using mainly CM techniques.The use of these techniques allows consumers to state their preference through the comparison of only a few different vehicles at a time.
Table 1 presents the CM studies selected for this review, allowing identification of some trends.First, the number of CM studies has increased significantly in the last six years, with almost three quarters of the studies appearing after 2010.Second, regarding the targeted consumers, North Americans were the most studied, followed by the Europeans.In the first two decades studies were exclusively from North America, but since 2000, European and Asian studies started to be developed (Figure 2).This trend was clearer in the 2010s when the number of European and Asian studies surpassed the North American.Third, three broad goals of CM studies were identified: to analyze consumer preferences, to forecast the vehicles demand and to develop methodologies for CM.The number of studies aiming at understanding consumer preferences has been markedly increasing relative to forecast studies in recent years (Figure 3).Fourth, the majority of studies were focused on a specific vehicle technology, mainly BEV (55%) or BEV plus other vehicles (36%) (Figure 4).Lastly, The second phase involved an analysis of the references cited by these studies in order to cover all the relevant papers, which resulted in the final selection of 79 studies that are analyzed in the next sections.

CM Studies
Understanding consumer preferences has become more complex because consumers have a wide range of choices, such as ICEVs vehicles (gasoline and diesel), BEVs, HEVs, PHEVs, Fuel Cell Vehicles (FCVs), Compressed Natural Gas Vehicles (CNGs), Natural Gas Vehicles (NGVs), and Biofuel vehicles.Therefore, consumers are constantly confronted with huge amounts of information about these vehicles, which is used by them to form preferences and, consequently, to make purchase decisions [34].This context has been justifying the interest of a substantial body of researchers that developed studies aiming at understanding consumer preferences concerning these vehicles using mainly CM techniques.The use of these techniques allows consumers to state their preference through the comparison of only a few different vehicles at a time.
Table 1 presents the CM studies selected for this review, allowing identification of some trends.First, the number of CM studies has increased significantly in the last six years, with almost three quarters of the studies appearing after 2010.Second, regarding the targeted consumers, North Americans were the most studied, followed by the Europeans.In the first two decades studies were exclusively from North America, but since 2000, European and Asian studies started to be developed (Figure 2).This trend was clearer in the 2010s when the number of European and Asian Energies 2019, 12, 318 5 of 33 studies surpassed the North American.Third, three broad goals of CM studies were identified: to analyze consumer preferences, to forecast the vehicles demand and to develop methodologies for CM.The number of studies aiming at understanding consumer preferences has been markedly increasing relative to forecast studies in recent years (Figure 3).Fourth, the majority of studies were focused on a specific vehicle technology, mainly BEV (55%) or BEV plus other vehicles (36%) (Figure 4).Lastly, considering the vehicles included in the stated preference surveys of CM studies over time, it was observed that the more recent studies compare a more diversified vehicles set than the older studies where only BEVs were compared with ICEVs (Figure 5).Additionally, it was observed that the most common set of vehicles compared were ICEVs, BEVs and HEVs.

Individual Characteristics of Consumers
AFVs are innovative and marketed as environmentally friendlier products.Therefore, these two dimensions of consumer behavior need to be addressed in order to understand what drives consumers' preferences for these vehicles.Several studies focused on which consumers would prefer environmental or innovative products.Consumer profiling for environmentally friendly products began in the 1970s [99] and was followed by studies attempting to identify which consumer specific characteristics could be related to the consumption of ecological products [99,100].Regarding the innovative dimension, an extensive literature has analyzed the consumer behavior about innovative products trying to understand the influence of personal characteristics on the adoption of innovative products [101].As the influence of consumers' characteristics on their preferences/behavior does not affect equally all the innovative and/or environmentally friendly products, this review aims to understand what are the trends, if any, of consumer personal characteristics are connected to preferences for AFVs.
Demographic variables are frequently and extensively analyzed in all consumer-based research [101] and have been considered one of the major influences on vehicle demand [30].The list of demographic characteristics that can influence consumer preferences is extensive.Therefore, for this analysis, the selection of individual characteristics was based on the relevant characteristics for innovative and environmental products found in previous studies.Laroche et al. [99] present the demographic characteristics that can influence purchases of environmentally friendly products namely, age, gender, income, level of education, employment status, home ownership, marital status and family size.Concerning the adoption of innovative products, Kaushik and Rahman [101] through an extensive literature review about consumer innovativeness, identified age, income, education, gender, sexual orientation, religion and family size as the most common characteristics related to consumer preferences.In order to analyze their influence on consumer preferences for AFVs, age, gender, income, level of education and family size were selected, which are common to the lists of Laroche et al. [99] and Kaushik and Rahman [101].Moreover, two vehicle-related demographics were added, driving habits and number of vehicles owned per household, due to their relevance and frequent analysis in AFVs studies.

Individual Characteristics of Consumers
AFVs are innovative and marketed as environmentally friendlier products.Therefore, these two dimensions of consumer behavior need to be addressed in order to understand what drives consumers' preferences for these vehicles.Several studies focused on which consumers would prefer environmental or innovative products.Consumer profiling for environmentally friendly products began in the 1970s [99] and was followed by studies attempting to identify which consumer specific characteristics could be related to the consumption of ecological products [99,100].Regarding the innovative dimension, an extensive literature has analyzed the consumer behavior about innovative products trying to understand the influence of personal characteristics on the adoption of innovative products [101].As the influence of consumers' characteristics on their preferences/behavior does not affect equally all the innovative and/or environmentally friendly products, this review aims to understand what are the trends, if any, of consumer personal characteristics are connected to preferences for AFVs.
Demographic variables are frequently and extensively analyzed in all consumer-based research [101] and have been considered one of the major influences on vehicle demand [30].The list of demographic characteristics that can influence consumer preferences is extensive.Therefore, for this analysis, the selection of individual characteristics was based on the relevant characteristics for innovative and environmental products found in previous studies.Laroche et al. [99] present the demographic characteristics that can influence purchases of environmentally friendly products namely, age, gender, income, level of education, employment status, home ownership, marital status and family size.Concerning the adoption of innovative products, Kaushik and Rahman [101] through an extensive literature review about consumer innovativeness, identified age, income, education, gender, sexual orientation, religion and family size as the most common characteristics related to consumer preferences.In order to analyze their influence on consumer preferences for AFVs, age, gender, income, level of education and family size were selected, which are common to the lists of Laroche et al. [99] and Kaushik and Rahman [101].Moreover, two vehicle-related demographics were added, driving habits and number of vehicles owned per household, due to their relevance and frequent analysis in AFVs studies.
Table 2 comprises the studies that collected consumer characteristics and presents the purpose of each study on collecting such data for consumer preferences analysis.Most of the studies collected consumer characteristics data (92%) where the most analyzed characteristic is age, followed by gender and income.Regarding the purpose of collecting such data, the main reasons identified were the analysis of sample representativity (52%), analysis of the interaction of individual characteristics with vehicle preferences (41%) and with vehicle attributes (15%) (Figure 6).Only one study (Mabit and Fosgerau [57]) covered these three analyses.It can be highlighted that 27% of the studies collected demographic data exclusively to analyze if the sample was representative.

Sample Size and Representativity
The results from the selected studies can be compared in order to find relevant conclusions.However, as the group of studies differ regarding the sample characteristics, a complementary analysis of the samples size and representativity was done in order to enhance the comparability of the results.This analysis comprised only the studies that collected consumer demographics data (Table 2).
Regarding the sample size, all the studies to be compared should have a sample size large enough to allow the drawing of reliable conclusions from the involved choice tasks.For each study, the minimum sample size, , was computed using a standard computation through an expression adapted from [102],  ≥ 500   , where  is the number of choice tasks that each consumer has to answer,  is the number of alternatives per choice task and  is the largest number of levels for any one attribute.The analysis allowed verifying that all the studies fulfilled this requirement.In consumer preference studies for AFVs the sample representativity is not usually set as a goal.This is mainly due to the use of convenience samples (used in 65% of the analyzed studies), i.e., most studies select consumers that meet specific requirements such as consumers that intend to purchase a vehicle in the next five years, which usually occurs at the cost of not getting representative samples of the targeted population.A low number of studies that aimed to have representative samples was found (53%) (see Figure 6) from which only 38% achieved samples that would represent the targeted markets.These findings support that sample representativity is not a priority in these types of studies, and for that reason is not an obstacle to the comparison of studies, although keeping in mind that these may not represent accurately the entire population of a country.

Sample Size and Representativity
The results from the selected studies can be compared in order to find relevant conclusions.However, as the group of studies differ regarding the sample characteristics, a complementary analysis of the samples size and representativity was done in order to enhance the comparability of the results.This analysis comprised only the studies that collected consumer demographics data (Table 2).
Regarding the sample size, all the studies to be compared should have a sample size large enough to allow the drawing of reliable conclusions from the involved choice tasks.For each study, the minimum sample size, n, was computed using a standard computation through an expression adapted from [102], n ≥ 500 c ta , where t is the number of choice tasks that each consumer has to answer, a is the number of alternatives per choice task and c is the largest number of levels for any one attribute.The analysis allowed verifying that all the studies fulfilled this requirement.In consumer preference studies for AFVs the sample representativity is not usually set as a goal.This is mainly due to the use of convenience samples (used in 65% of the analyzed studies), i.e., most studies select consumers that meet specific requirements such as consumers that intend to purchase a vehicle in the next five years, which usually occurs at the cost of not getting representative samples of the targeted population.A low number of studies that aimed to have representative samples was found (53%) (see Figure 6) from which only 38% achieved samples that would represent the targeted markets.These findings support that sample representativity is not a priority in these types of studies, and for that reason is not an obstacle to the comparison of studies, although keeping in mind that these may not represent accurately the entire population of a country.

Relation between Consumer Preferences for Vehicle Type and Demographic Data
In order to understand the direction in which the consumer characteristics influence consumer preferences for AFVs, several hypotheses were formulated based on what was found in the literature regarding the purchase behavior of consumers for sustainable and innovative products in general.Below we present these hypotheses and discuss whether the conclusions of studies focused on AFVs support them.

Age Influence
The effort to understand consumer preferences, and consequent behavior, of the market segments defined by the consumers age is very common [103].The relationship between age and the adoption of new environmental friendly products has motivated several studies pointing out that it is expected that younger consumers have higher preferences for innovative and/or environmentally friendly products for several reasons: younger consumers have higher propensity to try and adopt novel products, they are more likely to choose products that imply considerable changes to their daily routines and to take risks than older consumers [104]; consumers that grow up in a time period with higher environmental concerns are more sensitive to ecological issues and related products [100]; older consumers will try a new product only if they find that product will satisfy a specific need they have, not just because it is trendy [105].These considerations led to the development of the following hypothesis: H1.Younger consumers have higher preferences for AFVs.
Regarding BEVs, several studies concluded that younger consumers preferred these vehicles more [6,21,23,26,46,71,83,95] or that older consumers have lower preferences for BEVs or prejudice against these vehicles [20, 37,77].On the other hand, some studies concluded that older consumers are more likely to purchase BEVs [19,61], possibly because they can afford the higher initial cost to buy these vehicles and are less concerned about the limited range [19,61].Focused on FCVs, Ziegler [21] found that younger consumers have higher propensity to choose these vehicles.Concerning HEVs, Senturk et al. [59] reported that older consumers prefer these vehicles over gasoline vehicles, which can be justified by their higher sensitivity to the factors that affect negatively their health, whereas Hackbarth and Madlener [23] found that younger consumers are more likely to adopt HEVs.Additionally, there were studies that analyzed the effect of age on AFVs in general: one concluded that preferences for these vehicles increase with age [53] whereas others concluded that age affects negatively the preferences for AFVs [5, 16,58,78].
Since the results regarding the hypothesis H1 were inconclusive it was decided to seek other explanations.Cultural differences can lead to variations of consumers level of innovativeness [106]; for instance, North American consumers have reportedly higher propensity for innovative products than consumers from Asian countries [107].In this context, the following hypothesis was analyzed in order to unveil a potential explanation for the identified contrasts: H2.The influence of age on consumer preferences for AFVs varies across geographical regions.

Gender Influence
Consumer behavior varies according to gender, which is mainly justified by role differences in cultural and social contexts [108].A significant impact of gender in the consumption of sustainable products [109] and innovative products [108] has been observed.
In general, women are more likely to have higher preferences than men for sustainable products, due to their stronger attitudes, values and consciousness towards the environment [64,109].Regarding innovative products the gender influence is more context-dependent because it is highly dependent on the type of product.Since the gender effect on preferences for AFVs is not easily anticipated we analyzed the following hypothesis:

H3. Men have higher preferences for AFVs.
Regarding BEVs, while several studies found that men preferred these vehicles less than women [46,57,79,83,88,95], there was only one study that concluded that men preferred BEVs [87].On the other hand, there were two studies where no interaction between gender and BEVs preferences was found [21,61].The studies of Mabit and Fosgerau [57] and Ziegler [21] differ on their results about FCVs: the first found that men have lower preferences for FCVs than women while the second concluded the opposite.Concerning HEVs, several studies found that it is less likely that men will purchase these vehicles [53,74,87,93].Considering AFVs in general, Qian and Soopramanien [58] observed that men are not keen to adopt a green vehicle.
Summing up, with the exception Ziegler [21] and Liu and Cirillo [87], the results of previous studies do not support the defined hypothesis by revealing that women are more willing than men to follow and prefer sustainable vehicles.These results can be explained by the different ways that women and men face the technical limitations of AFVs, as women are less sensitive to limited range [37] and men have more concerns about the driving range and fueling infrastructure for BEVs in the short-term [46].

Income Influence
Income is considered a strong predictor of the adoption of innovative products even though no influence between income and consumer innovative behavior has been reported in some studies [34,110].The main argument is that for innovative consumers to try and buy new products, a minimum level of income is needed [111].Therefore, income is expected to relate positively with new products by decreasing the resistance to their higher price [106].Additionally, high income consumers are likely to be more environmentally conscious because consumers with higher income can bear more easily the higher costs arising from adopting ecological products [100].In this context, the following hypothesis was analyzed: H4.Wealthy consumers have higher preferences for AFVs.
High levels of income are commonly assumed to be related to high levels of education [61].It is thus expected that wealthy consumers are better informed about the advantages of AFVs and are more likely to prefer them [19,52,59], by valuating more their operation cost savings [72].However, this relation cannot be generalized to all AFVs, due to the presence of contradictory results in the studies reviewed.Some studies concluded that consumers with higher income present higher preferences for BEVs [19,24,61] but another study concluded that consumers with higher earnings are more opposed to BEVs [72,83].Hidrue et al. [6] and Ferguson et al. [93] concluded that income did not influence consumers' choice for BEVs.Regarding HEVs, on one hand some studies concluded that wealthy consumers have stronger preferences for these vehicles [5, 53,58,83,97] whereas others found that consumers who earn more have lower intentions to adopt HEVs [19,72] or that consumers with lower income prefer HEV [94].Bunch et al. [37] reported that as consumer income increases the level of environmental concerns decrease and, for that reason, preferences for gasoline vehicles are higher.
In summary, regarding the influence of income on consumer preferences for AFVs no trend can be found so far as no consensus has been verified regarding the studied vehicle technologies, which leads us to inconclusive results regarding hypothesis H4.

Level of Education Influence
It is expected that education positively affects the adoption of innovative products, because it gives consumers a broader perspective and renders them more into new ideas and products [106].There are studies where education was found to have no influence on innovative behavior [110,112] and studies where education was found to affect positively consumer innovative behavior [106,111].Regarding environmental products, several studies found a positive link between higher education and environmental concerns [100,113], and, when only individuals with the same environmental concerns are considered, those who are more educated may present higher awareness regarding the external effects of their consumption behavior and higher concerns about social welfare [113].This background led us to analyze the following hypothesis: H5.Highly educated consumers have higher preferences for AFVs.
Concerning AFVs, it was found that environmental concerns increase according to level of education [49,64].Almost all the reviewed studies are consistent in their findings regardless of the type of vehicle analyzed: consumers with a higher level of education are more likely to prefer and buy BEVs [6,23,24,37,44,79,93,97]; HEVs [5] and PHEVs [23,24].In line with these findings, Sheldon et al. [26] and Huang and Qian [25] found that less educated consumers have less preference for BEV and PHEV.Zhang et al. [61] is the only study presenting contrary results by finding that well-educated consumers are unwilling to buy BEVs in the short-term.A possible explanation pointed out in this study is that the less developed sector of electric vehicle industry in China leads to consumers with higher knowledge levels to be more familiar with the disadvantages of these vehicles and consequently do not purchase them in the short-term.
In summary, it can be concluded that results from previous studies support hypothesis H5.

Family Size Influence
The influence of the number of family members on the purchase of innovative products is expected to be negative because parents' attention is more focused inward rather than outward to innovations [106].On the other hand, families who have children are more willing to pay more for environmental products due to their concerns about the negative impact of a ruined environment on their children's future [99].Therefore, the impact of the family size on environmentally friendly Energies 2019, 12, 318 20 of 33 vehicles preferences it is not easily predictable.In order to understand what the relation between family size and AFVs purchase behavior is, the following hypothesis was formulated: H6.Consumers with larger families have higher preferences for AFVs.
The literature reveals that studies addressing the influence of the number of family members in the preferences for EVs reached the same conclusion: larger families are more willing to purchase BEVs [25,39,58,61,74,79] or a PHEV [25,26].This is in line with hypothesis H6.These findings suggest that perceived environmental benefits of purchasing a more sustainable vehicle may be significant for larger families.

Vehicle-Related Demographics Influence
Two vehicle-related demographics influences were analyzed, driving habits and number of vehicles owned per household.
Driving habits are mainly expressed by the average vehicle mileage driven annually, weekly or daily [18,19,43,59,67,72] or by the type of route that consumers use more often, city or intercity routes [5, 23,58].On one hand, the influence of driving habits on preferences for AFVs may favor AFVs over diesel or gasoline vehicles as the running costs of AFVs are usually lower.On the other hand, it may influence consumers to not prefer AFVs as the owners of these vehicles face more often limited range and fuel availability problems [18].Therefore, we analyzed the following hypothesis: H7.Short-distance travelers have higher preferences for AFVs.
Two studies concluded that consumers that drive long distances present lower preferences for AFVs which was justified by their limited range and the limited availability of fuel that may compromise charging or refueling these vehicles [5,58].On the other hand, Dimatulac and Maoh [84] found that long-distance consumers are more likely to purchase HEVs in order to save on the price of gas.Considering consumers that undertake mainly city routes, Hackbarth and Madlener [23] concluded that these consumers are more willing to buy BEVs due to the suitable range of these vehicles for city journeys.In this sense, the results tend to support hypothesis H7 in that the influence of driving habits on preferences for AFVs is highly related to the technical limitations of these vehicles.However, as manufacturers are continuously trying to overcome these limitations the influence of driving habits on consumer preferences may decrease over time.
The number of vehicles owned per household is expected to positively affect the willingness to buy AFVs because these vehicles are considered to be fuel efficient [59] and also because households with more than one vehicle can manage the limitations of some AFVs.The low range of BEVs, for instance, is less of a concern as they have other vehicles for their long-distance journeys.In this context, the following hypothesis was analyzed: H8.Consumers with more than one vehicle have higher preferences for AFVs.Some studies concluded that families that own more vehicles are more willing to buy a BEV [61] or a biofuel vehicle [21].One explanation pointed out for these results is the assumption that households that own more vehicles are wealthier and for that reason can more easily afford the higher purchase price of AFVs [61] (although we found inconclusive results concerning Hypothesis H4).On the other hand, Senturk et al. concluded that households with more vehicles present lower preferences for HEVs [59].Therefore, the results show that the influence of the number of vehicles might be dependent on the type of vehicle or may be related to the families' wealth and for that reason the results regarding hypothesis H8 are inconclusive.

Relation between Consumer Preferences for Vehicle Attributes and Demographic Data
The vehicle attributes analyzed more often were purchase price, range, fuel consumption and acceleration (Figure 7), whilst the influence of age and gender on preferences was tested with greater frequency.However, very few studies found a statistically significant influence of demographics on preferences for vehicle attributes.
acceleration (Figure 7), whilst the influence of age and gender on preferences was tested with greater frequency.However, very few studies found a statistically significant influence of demographics on preferences for vehicle attributes.
Regarding the influence of gender and age on preferences for range previous studies found that women [16,73] and younger consumers [16,57] are more sensitive to range.On the other hand, [75] concluded that women are less sensitive to range.Additionally, women were also found to be less sensitive to purchase price [75], acceleration [5, 16,75], fuel consumption [75] and top speed [46] than men.Concerning the vehicle size, women and younger consumers have higher preferences for midsize vehicles, while men and older consumers prefer large vehicles [22,42].

The Portuguese Case
This new study was developed in Portugal in order to collect and analyze the consumer preferences for AFVs.The Portuguese government targeted a 5% share of AFVs in 2020 [114], but the efforts put in place to successfully mass introduce these vehicles in the market have not been as effective as expected.Most of the incentives from the government and suppliers took place between Regarding the influence of gender and age on preferences for range previous studies found that women [16,73] and younger consumers [16,57] are more sensitive to range.On the other hand, [75] concluded that women are less sensitive to range.Additionally, women were also found to be less sensitive to purchase price [75], acceleration [5, 16,75], fuel consumption [75] and top speed [46] than men.Concerning the vehicle size, women and younger consumers have higher preferences for midsize vehicles, while men and older consumers prefer large vehicles [22,42].

The Portuguese Case
This new study was developed in Portugal in order to collect and analyze the consumer preferences for AFVs.The Portuguese government targeted a 5% share of AFVs in 2020 [114], but the efforts put in place to successfully mass introduce these vehicles in the market have not been as effective as expected.Most of the incentives from the government and suppliers took place between 2010 and 2012, but this coincided with the financial crisis in Portugal, when the transport sector faced a sharp decrease of LDVs sales (Figure 8).Additionally, crossing the timing of the government incentives (consulted in www.dre.pt(electronic Diary of Republic)) for EVs penetration and the evolution of sales shows that that the sales dynamics did not respond to the incentives as would be expected; in fact, in some periods of time they behaved in the opposite direction (Figure 9).For instance, plug-in electric vehicles (BEVs and PHEVs) demand increased steeply in 2012 and in 2016 after the purchase subsidy and the ICEVs discards incentives decreased, respectively.Additionally, crossing the timing of the government incentives (consulted in www.dre.pt(electronic Diary of Republic)) for EVs penetration and the evolution of sales shows that that the sales dynamics did not respond to the incentives as would be expected; in fact, in some periods of time they behaved in the opposite direction (Figure 9).For instance, plug-in electric vehicles (BEVs and PHEVs) demand increased steeply in 2012 and in 2016 after the purchase subsidy and the ICEVs discards incentives decreased, respectively.Additionally, crossing the timing of the government incentives (consulted in www.dre.pt(electronic Diary of Republic)) for EVs penetration and the evolution of sales shows that that the sales dynamics did not respond to the incentives as would be expected; in fact, in some periods of time they behaved in the opposite direction (Figure 9).For instance, plug-in electric vehicles (BEVs and PHEVs) demand increased steeply in 2012 and in 2016 after the purchase subsidy and the ICEVs discards incentives decreased, respectively.This context of AFVs market dynamics in Portugal along with the absence of studies analyzing the influence of demographics on Portuguese consumer preferences brings relevance to develop a study in this market under the scope of the review previously presented, i.e., aiming at analyzing Portuguese consumer preferences for AFVs that collected data through a stated preference survey elicited through CM methods.
The selection of alternatives in this stated preference survey includes all the main AFVs currently available in the Portuguese market, namely BEVs, PHEVs and HEVs.For comparison purposes two ICEVs vehicles were also included, namely Diesel and Gasoline vehicles.Regarding the attributes This context of AFVs market dynamics in Portugal along with the absence of studies analyzing the influence of demographics on Portuguese consumer preferences brings relevance to develop a study in this market under the scope of the review previously presented, i.e., aiming at analyzing Portuguese consumer preferences for AFVs that collected data through a stated preference survey elicited through CM methods.
The selection of alternatives in this stated preference survey includes all the main AFVs currently available in the Portuguese market, namely BEVs, PHEVs and HEVs.For comparison purposes two ICEVs vehicles were also included, namely Diesel and Gasoline vehicles.Regarding the attributes selection and according to a previous study, purchase price, fuel consumption, range and CO 2 emissions, in this order, are the most relevant characteristics for consumers when differentiating similar vehicles with different powertrains [115].The type of engine was added to this list of attributes in order to distinguish the vehicle technology of each alternative.The attributes are defined as follows: -Purchase price: cost to acquire a vehicle, measured in €; -Range: distance that can be driven without fueling/charging the vehicle, measured in km; -Fuel consumption: cost to drive 100 km, measured in €/100 km; -CO 2 emissions: quantity of CO 2 emissions released to the environment during the usage phase of the vehicle, measured in g/km.
Two tasks were included in the survey.One task consisted in collecting data about consumers' characteristics and their vehicles, namely age, gender, level of education, current vehicle, main route, number of kilometers driven per year and knowledge about EVs.The other task comprised a set of Choice-Based Conjoint Analysis (CBC) rank-order questions where consumers were asked to choose the preferred and the least preferred alternative among a set of three.Previous to the design of CBC questions a set of levels was defined for each attribute (Table 3).The CBC questions were obtained through a fractional factorial experimental design, which combined all these attribute levels using Sawtooth ® software (Sawooth Software, Utah, UT, USA).As a result, eight versions of nine questions each were obtained that were randomly assigned to each consumer (105 respondents).The sample was drawn on a convenience basis allowing to gather data from a group of consumers with more interesting characteristics for the study purposes.The use of a convenience sample gives an exploratory nature to this work that aims to provide insights about the influence of demographics on Portuguese preferences for AFVs and not to be representative of Portuguese consumers.Two selection criteria were applied: consumers should be older than 18 years old and should be potential vehicle buyers in the short-medium term.Data was collected through face-to-face interviews where the SP surveys were presented for each consumer individually.

Analysis of Sample Representativity
According to Statistics Portugal from Census 2011 [116], the sample used is not representative of Portuguese population, as young adults, men and consumers with higher education are overrepresented (Table 4).The misrepresentation of Portuguese population is a downside of selecting a convenience sample that fits the selection criteria for the study.However, as previously mentioned, the absence of representativity is not a major concern if it allows gathering data from a group of consumers with more interesting characteristics for the purposes of the study.

Analysis of the Interaction of Demographic Data with Vehicle Type and Attributes
The analysis of interactions was made through counting analysis, namely through a "Between group Chi-Square" test.This test consists in identifying if the levels of one attribute significantly differ in their choice frequency between demographic groups, for example if women are more likely to prefer a BEV than men.
The counting analysis results are depicted on Tables 5 and 6, along with the respective Chi-Square results.Considering only the results that were found to be statistically significant for the "Between group Chi-Square" test, some conclusions could be derived.According to the results, preferences for the type of engine are frequently influenced by demographic characteristics of consumers.BEVs are more likely to be preferred by older consumers, similar to Zhang, Yu, et al. [61] and Shin et al. [19]; by consumers that drive less annually and by city drivers, in line with Hackbarth and Madlener [23].On other hand, younger consumers, drivers of intercity routes and consumers that drive less have higher preferences for gasoline vehicles.In line with findings in Dimatulac and Maoh [84], consumers that drive long-distances more often have higher preferences for HEVs.Regarding the interaction with vehicle attributes we tested the hypotheses of demographic variables influencing consumers' sensitivity to vehicle attributes variations.A few statistically significant results were found that indicate an influence of demographic factors on vehicle attribute sensitivity, namely: -Consumers with higher education are more price sensitive; -City drivers are less sensitive to range; -Older consumers and consumers that drive less are more sensitive to fuel consumption; -Lower knowledge consumers are less sensitive to lower emission values.

Main Conclusions and Discussion
To the authors' best knowledge, this is the first in-depth review focused on the influence of consumer demographics on preferences for AFVs and their attributes.It summarizes the research paths that have been treaded during the last decades, providing insights on the main trends in AFVs market studies.This study also adds to the literature by analyzing the influence of demographic characteristics on preferences of Portuguese consumers.
Considering the target markets for consumer preferences research, the review showed an increasing number of studies in European and Asian countries.When specific vehicle technologies are focused, BEVs increasingly continue to attract interest from researchers worldwide.
Identifying the influence of consumer demographics in the consumer willingness to buy AFVs helps to understand the influence of such factors in the acceptability of those technologies [117] and it supports the market penetration strategies for these vehicles by policy-makers [17].In this sense, the present review of the influence of consumers' characteristics on their preferences provided some directions that can be used in future policies implementation.Seven individual characteristics were selected for a thorough analysis: age, gender, income, level of education, family size, driving habits and number of vehicles owned per household.Their influence was analyzed according to the vehicle technologies.As the influence of consumers' age revealed contradictory results regarding all technologies, a complementary analysis considering the consumers geographical region was made in order to analyze if more consistent outcomes were found.This revealed that younger consumers have stronger preferences for AFVs in European and North American studies, whereas older consumers in Asia are more willing to buy these vehicles.Considering the consumers' income influence, no consistent results were found.Regarding the other demographic characteristics, it was verified that consumers with higher education levels, women and consumers with larger families have higher preferences for AFVs.
Knowing which consumers segments have higher propensities for AFVs may support strategic decisions of vehicle manufacturers concerning the introduction of these vehicles in the market, such as marketing campaigns focused on those segments.In addition, policy-makers can use that information to support policy design aiming at increasing the market penetration of AFVs.Regarding the vehicle-related demographics, it was observed that the influence of driving habits on AFVs preferences is linked to the technical limitations of these vehicles, mainly limited range and fuel availability.The analysis of the influence of the number of vehicles owned showed that it may depend on the vehicle type and suggests that crossing it with the wealth of the households could help to explain their influence on consumer preferences.
Regarding the study of preferences in Portugal, the results provided several insights about the influence of demographic variables on vehicle choice and on sensitivity to vehicle attributes.The results showed that demographic variables frequently influenced preferences for the type of vehicles chosen, mainly, age, type of route and annual distance driven by consumers.Acknowledging the lack of information from previous studies concerning significant relationships between demographic characteristics and preferences for vehicle attributes, this study presents new insights about vehicle attributes sensitivity, namely the influence of age and annual distance on fuel consumption, the influence of route on range and the influence of knowledge on CO 2 emissions.Among the relationships found between demographics and vehicle attributes preferences, there were two which had an unexpected direction, namely the higher price sensitivity from high educated consumers and the higher sensitivity to fuel consumption from consumers that drive less.Concerning the first relationship, it can be considered counterintuitive because higher educated consumers tend to be wealthier and therefore less sensitive to price.We conjecture that this might not be the case among our convenience sample, which included many young Portuguese with college degrees, but who nowadays often earn less than older consumers without a degree.Another possible explanation is that better numeracy leads higher educated consumers to be more attentive to cost implications.Concerning the second relationship, one would expect that consumers driving less can afford a higher cost per km.A possible explanation for the relationship found is that consumers avoid driving, or drive less, when fuel price is higher due to their higher sensitivity to higher driving costs, but this relationship should be further examined in future studies.No relationship was found regarding the influence of gender on vehicle attributes preferences, contrary to previous studies that found significant relationships between gender and several attributes.

Research Gaps and Recommendations for Future Research
Throughout this review three main gaps have been identified which should be addressed in future studies.One gap concerns to the lack of studies focused on consumer preferences for PHEVs.Given that PHEVs are a combination of BEVs and conventional vehicles, they offer the comfort of a "safe ground" from their fuel-based component that reduces the known range anxiety from pure electric vehicles and, at the same time, they also imply a change of habits to consumers through the need of plugging in to charge the vehicle batteries.Consequently, by increasing consumers' familiarity with the electric-related habits, PHEVs may act as a transitional technology for BEVs, by attenuating the consumers resistance to BEVs, similarly to the transitional role of HEVs for PHEVs adoption found in the US [118].In this context, identifying the characteristics of consumers that are more willing to buy these vehicles could support strategies to increase the market penetration of PHEVs, and later BEVs.
Another gap is related to the geographical scope of the analyzed studies.There are several countries whose governments have made efforts to effectively introduce electric vehicles, namely France, Sweden, Portugal, Italy, Greece [119].However, research on consumer's preferences in these countries is still lacking in order to understand which consumers segments have higher propensities to adopt AFVs.The present study contributed with insights for the Portuguese market, but future studies should address the remaining unstudied markets to support the existent strategies to increase AFVs circulation or to help design more effective ones.
The third and final gap was the lack of studies that analyze the influence of consumer demographics on preferences for vehicle attributes.Only a few studies (15%) analyzed this interaction and very few found any significant relationship.Therefore, there are plenty relationships between individual characteristics and vehicle attributes to be addressed, e.g., age vs. range or route vs. fuel consumption.This information could be valuable for future AFVs promotion strategies.
Considering the summarized results of this study some recommendations for future consumer preference studies can also be made.One suggestion comes from the findings of the age influence analysis on consumer preferences for AFVs, where it was necessary to cross two consumer characteristics (age and geographic area) to identify trends on consumer preferences for AFVs.Therefore, for future studies, we suggest the analysis of interactions between individual characteristics in order to verify if relations can be identified.A specific suggestion comes from one of the highest concerns about BEVs, the limited range, which can be surpassed if consumers have a second vehicle for long-distance journeys.Therefore, as wealthy consumers probably have more than one vehicle, we suggest an analysis focused on the interaction between consumers' income and willingness to buy BEVs as primary and as second vehicles.Given the inconclusive results about the income influence on preferences for AFVs more research is needed to clarify the willingness to buy AFVs by wealthy consumers.
The overall results of the demographics influence on preferences for AFVs of this review, by identifying a frequent positive or negative influence of each demographic factor on preferences, stress a recommendation for future studies to include a segmentation analysis which takes into account the consumer preferences heterogeneity.This review provides some insights about which segments should be considered as a starting point, such as highly educated women or larger families with several vehicles.The methodological approaches of the most recent studies in this review underline the importance of targeting specific groups of consumers by including latent class models that allow the identification of market segments with similar preferences [17,76,78].

Energies 2018 ,
11, x FOR PEER REVIEW 4 of 35 the abstracts followed by the full text reading of the remaining 86 studies.As a result, an initial selection of 60 studies was obtained.

Figure 1 .
Figure 1.Diagram describing the selection strategy of studies.

Figure 1 .
Figure 1.Diagram describing the selection strategy of studies.

Figure 3 .
Figure 3. Goals of CM studies evolving with time.

Figure 3 .
Figure 3. Goals of CM studies evolving with time.Figure 3. Goals of CM studies evolving with time.

Figure 3 .
Figure 3. Goals of CM studies evolving with time.Figure 3. Goals of CM studies evolving with time.

Figure 3 .
Figure 3. Goals of CM studies evolving with time.

Figure 5 .
Figure 5. Vehicles included in the CM studies.

Figure 6 .
Figure 6.Purpose of collecting demographic data.

Figure 6 .
Figure 6.Purpose of collecting demographic data.

Figure 7 .
Figure 7. Frequency of vehicle attributes analyzed more often.

Figure 7 .
Figure 7. Frequency of vehicle attributes analyzed more often.

Energies 2018 ,
11, x FOR PEER REVIEW 23 of 352010 and 2012, but this coincided with the financial crisis in Portugal, when the transport sector faced a sharp decrease of LDVs sales (Figure8).

Figure 8 .
Figure 8. Evolution of Light Duty Vehicles (LDVs) sales with AFVs models introduction and the achieved AFVs share.

Figure 8 .
Figure 8. Evolution of Light Duty Vehicles (LDVs) sales with AFVs models introduction and the achieved AFVs share.

Figure 8 .
Figure 8. Evolution of Light Duty Vehicles (LDVs) sales with AFVs models introduction and the achieved AFVs share.

Figure 9 .
Figure 9.Sales of plug-in electric vehicles crossed with Portuguese government incentives for electric mobility (IUC = circulation tax; ISV = vehicle purchase tax).

Figure 9 .
Figure 9.Sales of plug-in electric vehicles crossed with Portuguese government incentives for electric mobility (IUC = circulation tax; ISV = vehicle purchase tax).

Table 1 .
Consumer preference studies for Alternative Fuel Vehicles (AFVs).

Table 2 .
Demographic characteristics analyzed in consumer preference studies (NM: Not Mentioned).

Table 3 .
Levels for each attribute.

Table 4 .
Demographics of consumers.

Table 5 .
Counting analysis for each consumer characteristics.

Table 6 .
Counting analysis for each consumer characteristics (cont).