The construction of highway suspension bridges has entered a new era, with the main span now exceeding 2300 m. However, the vertical stiffness design of such super-long-span structures remains a critical challenge, particularly regarding the selection of appropriate stiffness indices and the determination of their permissible limits. This study establishes an evaluation framework that integrates vehicle-bridge interaction (VBI) analysis to account for safety and comfort requirements with a systematic method for evaluating key stiffness indicators. Applying this framework to the 2300-m Zhangjinggao Bridge, the results demonstrate that local dynamic comfort at the girder ends governs the structural design. The results show that deflection, curvature, and girder-end rotation are strongly correlated, with Spearman correlation coefficients exceeding 0.92 within the investigated range. Among them, the maximum girder-end rotation is identified as the governing serviceability indicator. By combining the comfort threshold of 0.315 m/s² with the vehicle-bridge interaction results, a permissible girder-end rotation limit of 0.025 rad is proposed.