Improving poor subgrades is essential to facilitate highway construction and ensure long-term stability. Improvements include enhanced load distribution by using crushed rock "capping" layers or increasing the subbase thickness if the bearing capacity of the subgrade remains unchanged, and lime stabilization of soft clay subgrades improves the subgrade stiffness and strength to reduce or remove the need for additional crushed rock. However, the relative sustainability of subgrade improvement options as determined by indicators such as carbon dioxide (COD emissions is not immediately apparent. If capping is used, the relative depth requirements, together with variations due to different aggregates, make the lowest emissions option unclear. Lime production incurs far greater CO2 emissions than aggregates production, but it is used in smaller quantities and the quantity truly required, as opposed to that commonly specified, requires consideration. To address these decision-making dilemmas, a methodology is presented that allows the relative sustainability, according to CO2 emissions, to be evaluated. This methodology is illustrated through analysis of available improvement options for a typical low California bearing ratio clay subgrade by using capping, subbase thickening and lime stabilization, or a combination of these solutions. It is concluded that analysis of the relative sustainability of subgrade improvements can be informed by emissions calculations and that the lowest emissions option is heavily dependent on associated highway haulage vehicle movements. However, wider appreciation of financial, social, and environmental factors and a fundamental understanding of the role of lime stabilization are necessary if road foundations are to be constructed sustainably.