Transport policy research has been developed systematically since the second half of the 20th century, when urban planning and transportation systems became critical components of economic development and urbanization. The field has evolved through a wealth of diverse publications and scholarly contributions. Our comprehensive guide synthesizes the state of the art in 20 major categories of transport policy research, identified based on a data-driven categorization. Each category is represented by a leading expert in the field, providing a rigorous assessment of the state of the art on an area together with an identification of challenges to inform future research and policymaking. Aggregated analysis reveals six major trends in policy research, namely: Adaptability, Network Integration, Sustainability, Wireless Connectivity, Equity, and Resilience — combined to form the word ANSWER, with each component being crucial to understanding and addressing the field's multifaceted challenges. Our guide aims to provide answers to pressing challenges and serve as an essential, accessible reference regarding transport policy research for academics, policymakers, and industry stakeholders alike.

The transition process towards renewable energy systems is facing challenges in both fluctuating electricity generation of photovoltaic and wind power as well as socio-economic disruptions. With regard to sector integration, solutions need to be developed, especially for the mobility and the industry sector, because their ad hoc electrification and decarbonization seem to be unfeasible. Power-to-fuel (P2F) technologies may contribute to bridge the gap, as renewable energy can be transferred into hydrogen and hydrocarbon-based synthetic fuels. However, the renewable fuels production is far from economically competitive with conventional fuels. With a newly developed agent-based model, potential developments in the German energy markets were simulated for a horizon of 20 years from 2016 to 2035. The model was constructed through a participatory modeling process with relevant actors and stakeholders in the field. Model findings suggest that adjusted regulatory framework conditions (e.g., exemptions from electricity surtaxes, accurate prices for CO2-certificates, strong start-up subsidies, and drastic emission reduction quotas) are key factors for economically feasible P2F installations and will contribute to its large-scale integration into the German energy system. While plant capacities do not exceed 0.042 GW in a business-as-usual scenarios, the above-mentioned adjustments lead to plant capacities of at least 3.25 GW in 2035 with concurrent reduction in product prices.