With the goal of “achieving carbon peak by 2030 and carbon neutrality by 2060” proposed by China in 2020, energy conservation and carbon reduction in the transportation industry have become particularly important. As an established industrialized country, Britain started its research on carbon emission reduction in the transportation sector relatively early. This paper compared and analyzed the similarities and differences between the highway carbon emission accounting systems of China and Britain, so as to explore their respective policy backgrounds, accounting methods, and application effects and provide relevant suggestions for highway carbon emission reduction in China. Through literature analysis and case studies, the policy frameworks and implementation mechanisms of carbon emission accounting in the two countries were systematically reviewed. The results indicate that China focuses on government-led regulation and monitoring and ensures data accuracy through laws, regulations, and monitoring systems; meanwhile, Britain adopts a market-oriented flexible mechanism to encourage multi-party participation and voluntary emission reduction. This difference leads to China being rigorous in setting and implementing emission reduction targets but lacking flexibility, whereas Britain demonstrates greater adaptability in practice. Based on this, the following suggestions are proposed: China should comprehensively evaluate its accounting modules, clarify medium-term emission reduction targets, and refine them into annual operable targets; renewable energy policies should be promoted in the energy sector to reduce dependence on high-emission energy; in the transportation sector, the promotion of electric vehicles should be continuously increased, and public transportation networks should be improved. The research results can provide a useful reference for China to optimize the highway carbon emission accounting system, emphasize the importance of international cooperation and knowledge sharing in promoting low-carbon technology innovation, and provide a new perspective for the improvement of future carbon emission reduction policies and practices.