Energy vulnerability caused by car travel costs among French households: comparing a static and dynamic approach

With increasingly widespread car ownership, the car has become a life necessity, raising new issues of social inequality, car dependence and energy vulnerability (Dodson and Sipe, 2007). This risk has been intensified through the inappropriate spatial distribution of social groups with respect to transport costs (Orfeuil and Polacchini, 1999) and higher fuel prices. It has also become more sensitive politically in the context of fiscal reforms which led public authorities and researchers to assess the socioeconomic outcome of these policies, in addition to their environmental impact, notably for households and geographical areas more dependent on fossil fuels. However, energy expenditure is only one of several components when accounting for car travel costs. One of the most striking trends behind growing car costs is the increasing weight of compulsory expenditure, in the absence of anti-inflationary mechanisms. As a result, households can mainly make use of variable costs to moderate their travel budgets, which might be an incentive to reduce their mobility. Trade-offs might also exist between transport costs and other budget components (e.g., Coulombel, 2018), especially as housing has become much less affordable in the largest metropolitan areas. It is therefore a matter of concern to determine which indicators are more relevant to measure household vulnerability. Though using the energy budget share may be a primary approach (e.g., Nicolas et al., 2012), it may also show some limits, as it doesn't account for other budget components. Among alternative approaches, one can extend the budget share criterion to all car costs, or even identify what remains to the household for a living once all compulsory expenditure has been deducted from the available income, in order to focus on the most critical vulnerability situations. When it comes to energy costs, one may also account for "total residential cost", accounting for both transport- and housing-related energy costs (Maresca and Mercurio, 2014; CEREMA, 2016). In addition to the nature of costs considered, the analysis of vulnerability might disentangle "essential" from "non-essential" needs, or distinguish between critical and moderate vulnerability, depending on the extent to which transport costs impact budgets. However, an inconvenience of all the aforementioned indicators is their static nature, which does not account for adaptive behaviors caused by changing economic conditions, often synthesized through the concept of elasticity (e.g., Calvet et Marical, 2011). In this study, we compare results from a dynamic approach to energy vulnerability, accounting for adaptive behaviors, with those obtained through the conventional static method, where behavior is assumed to remain unchanged, using French Car Fleet Surveys. The analysis is categorized by number of adults, in order to account for heterogeneous choice sets, and by type of residential area, in relation to their variable degree of car dependence (e.g., Dargay, 2002). Under the common assumption of doubling fuel prices, different scenarios are considered, depending on income growth and vehicle efficiency gains. Of the various scenarios, the most beneficial are those where rising fuel prices are balanced with efficiency gains, making it possible to contain vulnerability within initially vulnerable groups. In contrast, scenarios without technical progress result in increased vulnerability, additionally extending it to new social groups and residential areas. Though adjustment strategies help mitigating long-term vulnerability, this still remains much higher than in the initial case. Most significant impacts are found among middle-class households and residents of smaller urban and rural areas, in consistency with the origins of popular protests against environmental pricing measures. These results highlight the necessity to stimulate technological research in order to accelerate efficiency gains and to implement selective financial assistance to households - either in order to purchase more efficient vehicles or to mitigate the impact of rising fuel prices on energy budgets - while redirecting the funding of transit networks towards small towns and developing mobility as a service for rural areas.