Cumulative Effects as a Decision‑Forcing Analysis: Lessons from Six U.S. Federal Infrastructure Projects

Cumulative effects assessment (CEA) is widely recognized as both central to and persistently weak in environmental impact assessment, yet few studies test whether project outcomes would plausibly have differed without it. This article assembles counterfactual evidence for a defined class of U.S. federal actions—major, contested infrastructure and resource development projects where cumulative effects substantially exceed direct impacts—using six cases spanning 1962–2025. Employing a comparative retrospective design, the analysis triangulates regulatory precedents for comparable projects, documentary evidence from environmental impact statements, records of decision and court opinions, and post‑decision trajectories where harms emerged or were averted. It asks whether direct‑effects‑only analysis would have supported approval and whether CEA altered project trajectories by revealing system‑scale risks. In all six cases, direct effects resembled those of projects that received federal authorization. In five, CEA—addressing watershed‑scale contamination, induced development, lifecycle greenhouse gas emissions, or synergistic ecological change—provided the analytical basis for denial, cancellation, or substantial constraint; in the sixth, the absence of CEA allowed basin‑wide degradation that later required extremely costly restoration. Across cases, the strength of documentary links between CEA findings and altered outcomes ranges from explicit legal citations to strong inferential support. These findings indicate that for contested projects where cumulative effects substantially exceed direct impacts, CEA has functioned as a necessary, though not independently sufficient, analytical condition for constraining system‑scale harm, clarifying what is at stake when cumulative analysis is weakened or removed from environmental review.