This paper investigates the impact of hydrodynamic obstruction on the runaway behavior of bubble walls in cosmological first-order phase transitions (FOPTs). The authors compare the maximal hydrodynamic obstruction, a frictional force that exists in local thermal equilibrium, with the Bödeker-Moore (BM) thermal friction, which is typically used to determine whether a bubble wall can run away. They derive an analytical expression for the maximal hydrodynamic obstruction and find that it can be larger than the BM thermal friction in large regions of the parameter space. This suggests that the conventional criterion for the runaway behavior of bubble walls may need to be modified. The authors also provide a modified runaway criterion that takes into account both the maximal hydrodynamic obstruction and the BM thermal friction. They derive critical phase transition strengths for both criteria and compare them, finding that the maximal hydrodynamic obstruction can become more relevant than the BM thermal friction for certain values of the ratio of degrees of freedom in the broken and symmetric phases. The paper includes a discussion on the validity of the approximations used and provides a fit for the critical phase transition strength in the LTE regime.This paper investigates the impact of hydrodynamic obstruction on the runaway behavior of bubble walls in cosmological first-order phase transitions (FOPTs). The authors compare the maximal hydrodynamic obstruction, a frictional force that exists in local thermal equilibrium, with the Bödeker-Moore (BM) thermal friction, which is typically used to determine whether a bubble wall can run away. They derive an analytical expression for the maximal hydrodynamic obstruction and find that it can be larger than the BM thermal friction in large regions of the parameter space. This suggests that the conventional criterion for the runaway behavior of bubble walls may need to be modified. The authors also provide a modified runaway criterion that takes into account both the maximal hydrodynamic obstruction and the BM thermal friction. They derive critical phase transition strengths for both criteria and compare them, finding that the maximal hydrodynamic obstruction can become more relevant than the BM thermal friction for certain values of the ratio of degrees of freedom in the broken and symmetric phases. The paper includes a discussion on the validity of the approximations used and provides a fit for the critical phase transition strength in the LTE regime.