*1.1. Motivation of the Research*

Road freight transport accounts for a significant share of global energy consumption in the transport sector. Almost 45% of global transport energy consumption corresponds to freight transport, with heavy-duty vehicles (HDVs) using more than half of that energy [1]. Furthermore, road freight transport depends heavily on fossil fuels; with medium freight trucks and heavy freight trucks accounting for 24% of global oil-based fuel consumption [2]. Diesel is the main fuel used in road freight transport, accounting for 84% of all oil products used; and corresponding to half of the global diesel demand [3].

Road freight vehicles are difficult to characterize due to the variety of vocational uses that requires a large number of sizes and configurations. However, it is considered that goods delivery from production sites to factories, industries and/or final retailers is one of the essential roles of road freight vehicles [3]. Road freight vehicles consist of light-duty vehicles (LDVs), medium-duty vehicles (MDVs) and HDVs; including but not limited to trucks. One additional difficulty for road freight vehicle characterization is that criteria for vehicle size classification vary depending on the country or region.

Despite the small share in road vehicles, MDVs and HDVs contribute disproportionally to mobile greenhouse gas (GHG) and air pollutant emissions and fossil fuel use, due to high vehicle fuel consumption, large annual traveled distances and long idling times [4]. In the European Union, HDVs account for 30% of on-road CO2 emissions, despite representing only 4% of the road vehicle stock [5]. Similarly in the United States, MDVs and HDVs account for 26% of transport CO2 emissions [6]. Additionally, road freight vehicles produce half of particulate matter (PM) emissions and one third of NOx emissions of the transport sector in cities [7]. In the United States, MDVs and HDVs are responsible for 22%, 28% and 24% of transport sector PM2.5, NOx and CO emissions, respectively [4]. PM and NOx are associated with adverse health effects in human beings [8]; while exposure to low CO concentrations causes health problems such as headaches and lightheadness [9]. Furthermore, diesel exhaust gas is classified as carcinogenic to humans (Group 1) by the World Health Organization (WHO) [10].

Decarbonization in road freight transport will be difficult to realize [11]. Global road freight activity is expected to grow in the future, driven by economic development. For instance, heavy-duty truck use is expected to increase 2.7%/year between 2000 and 2030 [12]. Increased road freight activity and high dependence on fossil fuels will cause GHG emissions from road freight transport to keep growing in the future; requiring coordinated efforts by shippers, logistics service providers and policymakers to mitigate their growth [2].

Potential for energy consumption and CO2 emissions reduction through the improvement of internal combustion engine efficiency and aerodynamics is significant and can be cost-effective [13]. However, in the long-term, advanced biofuels and electric-drive vehicles (EDVs) will be the main option to achieve deep decarbonization in road freight vehicles [14]. Additionally, since the time scales involved in road freight vehicle stock turnover are long, it is necessary to consider the dynamics of technology diffusion in the vehicle fleet when assessing the role of powertrain electrification in the decarbonization of road freight transport.

Compared with passenger LDVs, deployment of EDVs in road freight vehicles is still at an early stage. While the global EDV stock in LDVs exceeded 5.1 million vehicles in 2018, EDV stock in the fleet of light commercial vehicles reached 250000 vehicles; and medium-sized trucks sales were in the range of 1000 to 2000 vehicles [15]. Considering that global road freight vehicle stock totaled approximately 186 million vehicles in 2015 [3], penetration of EDVs in road freight transport is almost negligible.

Nevertheless, EDV deployment in road freight vehicles is gaining momentum, with McKinsey projecting that EDVs can account for 15% of global truck sales by 2030 [16]. Currently, several Original Equipment Manufacturers (OEMs) are investing in the development of EDVs for road freight transport. BYD introduced a battery electric class 8 truck and a battery electric refuse truck in 2017 [17]. Isuzu presented the battery electric ELF EV truck at the 2017 Tokyo Motor Show [18]. Tesla announced plans to manufacture battery electric semi-trucks by 2019, with UPS pre-ordering 125 vehicles [19]. Renault plans to start selling battery electric 12–16 ton trucks to complement the line-up of battery electric 4.5 ton trucks in the market since 2010 [20]. Mitsubishi Fuso started mass production of the battery electric eCanter in 2019 [21].

Regarding FCEVs, a Swiss consortium with participation of ESORO announced the development of a 34 ton fuel cell truck [22]. Toyota is working on a fuel cell system for heady-duty trucks to be used at the port of Los Angeles, United States [23]. Ballard and Kenworth have been testing a class 8 drayage fuel cell truck in the United States [24]. Nikola Motors plans to start selling Class 8 fuel cell trucks by 2021 [25].

Compared with passenger LDVs, EDV adoption in road freight transport has several advantages, especially at the early stages of deployment. Under some operating conditions, high annual traveled distances for long-haul vehicles enable fast recovery of the incremental capital costs through lower fuel costs [26]. In the case of BEVs, road freight vehicles follow fixed driving routines based on their delivery routes, making range anxiety concerns less critical [27]. Furthermore, operation according to a predetermined mobility demand makes the requirement for public charging modest [28]. In the case

of FCEVs, known daily traveled distance and central refueling station use make fleet-based operation attractive [4].

Nevertheless, EDV deployment in road freight transport also face barriers, such as diversity in terms of vehicle sizes and vocational uses [4], larger weight and longer traveled distances [12], and larger emphasis on economic profitability and reluctance to take risks regarding new automotive technologies [29]. Even though several studies have demonstrated that under some operating conditions EDVs are cost-effective [30–32], road freight vehicle fleet owners often focus on short payback times rather than cost-effectiveness over the vehicle life cycle, which can affect EDV adoption. For example, in the United States, the average payback time for large heavy-duty truck fleets is 24 months [33]; while payback times as short as 6 months are reported for small heavy-duty truck fleets [34].
