Increasing quantity and quality of roads are pathways to sustainable road infrastructure development. Understanding quantity-quality relations of road sustainability is critically required for strategic decision making. However, few knowledge are available about assessing quantity-quality relations of road sustainability from a perspective of spatial disparities. Here, we developed a Spatial Trade-Off Relation (STOR) model for assessing road quantity-quality trade-off relations at 42,425 blocks in Western Australia (WA). First, a sustainable road infrastructure index (SRII), including quantity and quality phases, was developed regarding stakeholder requirements and using multiple spatial methods to examine block-level road sustainability. Next, quantity-quality trade-off relations for road sustainability was investigated using a diminishing marginal utility approach. Further, spatial disparities of quantity-quality trade-offs were assessed through the spatial clustering based identification of hotspots and cold spots in trade-offs. Finally, contributions of the road quantity-quality interaction to economic development were estimated with the consideration of non-linear and geographically local characteristics of the associations using a generalized additive model and geographically weighted regression. We found three stages of the quantity-quality relations of road sustainability, including the increasing, marginal, and negative returns. The increasing return revealed the simultaneous growth of quantity and quality in outer and remote regions, and marginal and negative returns were primarily located in major cities. In addition, regional disparities of the trade-offs were found from the identified blocks, towns and villages, where quantity and quality were spatially clustered, for informing priorities of future strategic decisions. We also found that the contribution of road quality was about three times the contribution of quantity to resident income. This study demonstrated that efforts regarding regional quantity-quality trade-offs were required to achieve global sustainable infrastructure development goals.