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How far it travels before that happens decides the safety of pipelines, mines and reactors, and it cannot yet be computed reliably.","posed_since":"","precise":"For a premixed mixture in a tube of diameter $D$, smooth or with obstacles of blockage ratio BR, predict the run-up distance $X_{\\mathrm{DDT}}/D$ and the location and mechanism of the final transition (hot-spot ignition through a gradient of ignition delay, shock-flame interaction, boundary-layer and wall effects) as functions of mixture reactivity, expansion ratio and $D$. An answer is a validated prediction without constants fitted to the same experiments.","problem_ref":null,"references":"","settled_by":"Blind predictions of $X_{\\mathrm{DDT}}$ for a set of mixtures and tube geometries, compared with new experiments.","status_note":"Resolved simulations of hydrogen-air DDT in channels reproduce experiments qualitatively; no quantitative run-up prediction across mixtures as of 2026 (not re-verified for 2024-2026).","title":"Predicting run-up distance to detonation in smooth and obstructed 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