Highway construction cost escalation poses a persistent challenge for transportation agencies, yet the component-level structure of this growth remains poorly understood. This study analyses 91 quarterly observations of the U.S. National Highway Construction Cost Index (NHCCI) from 2003 Q1 to 2025 Q3, applying component-level contribution analysis, volatility ranking, pre- versus post-2020 comparison, and ARIMA time-series modeling. The aggregate NHCCI grew by 230.8% over the study period, with 68.0% of that growth occurring in just 23 quarters after 2020. While asphalt was the dominant long-run contributor, the post-2020 period revealed a fundamentally different pattern ; traffic control (+468%), concrete (+341%), and electrical (+292%) recorded the largest proportional increases, indicating that recent cost escalation extended well beyond petroleum-linked materials and became broad-based across all major pay-item groups. ARIMA(3,1,3) forecasting projects near-term stabilisation of the index between 3.29 and 3.40 through 2026 Q3. The central message of this study is that infrastructure agencies can no longer rely on aggregate escalation factors alone. Cost forecasting, contingency allocation, and price adjustment clause design must be grounded in component-specific evidence , because in the post-2020 environment, the risk is everywhere, not just in asphalt.