The transport sector significantly contributes to environmental degradation, with road transport responsible for approximately 70 % of the sector's greenhouse gas emissions, which is only one of several externalities generated by transport. The transboundary nature of international mobility further complicates national emissions inventories and accountability mechanisms. Policies based on the polluter-pays principle, such as carbon taxes and cap-and-trade systems, aim to mitigate these impacts but are hindered by inconsistent metrics and reporting. This study proposes a novel taxation framework for transport-related externalities, addressing how comprehensive sustainability reporting, supported by standardised metrics and transparent tracking, incentivises sustainable practices and enhances transport sustainability. The methodology comprises three phases: (1) a scoping review of transport externalities, mitigation strategies, and polluter-pays taxation; (2) development of a blockchain-based platform to transparently track transport-related pollution and mitigation actions across the entire supply chain using unified Environmental Impact Units (EIU). Companies can register fleets, fuels, and routes on the platform, which allocates pollution costs and issues blockchain-stored EIU certificates for transparent reporting; and (3) focus group discussions to evaluate stakeholder perceptions of the platform against the tax policy criteria of the American Institute of Certified Public Accountants (AICPA, 2017). Despite strong stakeholder support for fairness and transparency, challenges in data standardisation and scalability persist, particularly in less developed regions. This research introduces a blockchain-based platform where transportation companies can register their fleets, record the types of fuel used, and specify their routes. Based on the initial phase of the research, in which transport-related pollution is allocated across different stages of the supply chain, the platform will calculate the associated pollution levels based on the companies' inputs. The platform will then allocate the costs for pollution mitigation measures and issue a digital certificate for a Unified Environmental Impact Unit, reflecting the environmental impact of the company's operations. This certificate will be stored on the blockchain, providing a verifiable, transparent record that can be used by any supply chain for transparent reporting and auditing purposes.