*4.3. Scenario Description*

First, this study adopts Shared Socioeconomic Pathway 2 (SSP2) for socioeconomic assumptions (see Supplementary Materials). Table 1 gives the scenario settings in terms of the relative energy intensity of cars based on the reference case. Three additional scenarios are analyzed in this study, depending on the degree of improvements in energy intensity in all passenger cars (Figure 5). The reference assumes no improvement in energy intensity in the future. The 'Mod-EI' scenario is based on the energy intensity improvement rate applied by Ruffini and Wei [29]. The 'Better-EI' and the 'Best-EI' scenarios have an additional 10 and 20 percentage points of energy intensity improvement compared to the 'Mod-EI' scenario in 2050, respectively.


**Table 1.** Energy intensity improvement scenarios (unit: normalized).

Since GCAM does not account for the number of vehicles explicitly, a conversion of transportation service demand into the number of cars is required. As shown in Equation (13), the number of vehicles (*Veh*) can be calculated from transportation demand (*D*) multiplied by the inverse of the load factor (*L*<sup>−</sup>1) and the inverse of the VKT per vehicle (*V*<sup>−</sup>1). The load factor (*L*) and VKT per vehicle (*V*) are assumed to refer to references [12,25], respectively, for all technologies in this study.

$$\begin{array}{cl}Veh &= PKT \times \frac{VKT}{PKT} \times \frac{Veh}{VKT} \\ &= D \times L^{-1} \times V^{-1} \end{array} \tag{13}$$
