Reducing carbon emissions from highway construction is vital for mitigating climate change. Based on an expressway project in central China, this study conducted a fine-grained CO₂ emission accounting and analysis. The findings reveal that bridges and culverts rank first in emissions among all subprojects, emerging as a “carbon hotspot” with an exceptionally high emission intensity, followed by subgrade and pavement. Additionally, traffic safety facilities, accounting for 6.40% of total emissions, are also not negligible. Regarding emission sources, material emissions exhibit an extreme dominance at 89.44%, whereas off-road machinery and transport vehicles demonstrate distinct patterns of input-emission asymmetry and single-vehicle-type dominance, respectively. Consequently, this study delineates decarbonization pathways, quantitatively verifies the low-carbon efficacy of four mitigation technologies, and focuses on evaluating the environmental-economic dual feasibility of substituting core equipment with pure electric alternatives. The results demonstrate that the resin-bonded gravel sound barrier achieves an emission reduction rate of 73.26%; furthermore, under a 50% penetration rate, deploying 30-ton class EV dump trucks can cut transportation emissions by 9.98%, maintaining exceptional financial resilience even under a 30% plunge in diesel prices. This study provides a replicable, scientific decision-making basis for the deep decarbonization of the transportation infrastructure sector.