The greenhouse gas emissions from conventional fossil fuel vehicles have a significant impact on the environment. Compared to fossil fuel vehicles, the new energy vehicles result in greatly lower carbon emissions. Some countries have been providing considerable subsidies for the popularization of new energy vehicles in the recent years, which has been gradually reducing the total urban transportation emissions. However, the rigidity of the existing subsidy policies has limited both policy implementation and incentive outcomes. In this paper, we propose a blockchain-enabled theoretical system for carbon emission trading between new energy vehicles and fossil fuel vehicles. The system is designed for free-market emission trading to realize incentives for clean transportation behaviors by punishing fossil fuel vehicles and rewarding new energy vehicles. To better contribute a high-performance and low-cost structure, Ethereum 2.0 and Rollups are adopted for the development of the new system. A discussion is conducted to illustrate the design advantages and social impact of this theoretical system. In the end, we analyze the feasibility of the proposed system.