A review of fleet-based life-cycle approaches focusing on energy and environmental impacts of vehicles

Alternative vehicle propulsion technologies are being promoted to reduce energy consumption and environmental impacts in transportation. Life-cycle assessment (LCA) is often used to assess and compare the environmental impacts of these technologies, but, in its traditional form, it lacks the ability to capture the transient effects as new vehicles displace older vehicles in the fleet. Fleet-based life-cycle (LC) approaches - which combine the LCA methodology with fleet models that describe the stocks and flows associated with a class of products over time have been proposed to circumvent this issue. This article presents a critical review of the literature addressing fleet-based LC approaches by providing an overview of modeling approaches, its main applications, and an analysis of the key aspects underlying environmental and energy impacts of vehicle fleets (focusing on electrification pathways). Fleet-based LC approaches have been applied with different purposes (e.g., to model recycling processes, to assess trade-offs between manufacturing and use impacts; to optimize product service life). The issue of evaluating the impacts of introducing alternative technologies is appropriately addressed by a fleet-based LC approach, because it allows for the capture of displacement effects, technological improvements over time, and changes in background processes. Several studies have used such an approach to assess scenarios of evolution of the light-duty fleet. The main key aspects are: fleet penetration rate, electricity source, fuel economy improvements, and vehicle weight reduction. Emission reductions were found to be very dependent on the underlying assumptions. Reducing fuel consumption is one of the key ways to reduce fleet GHG emissions, but it needs to be combined with other measures, such as high penetration of advanced technologies, to bring about significant reductions. The electricity generation source has also a large impact on the fleet GHG emissions and increasing renewable energy penetration is key to reduce overall emissions.