Shock waves generated in forward-mode detonation-driven shock tubes are intrinsically unsteady. Application of detonation drivers in research facilities is fundamentally limited by the extent to which the test flow is made unsteady by the decaying shock wave. Numerical simulations of a simple model problem are used to study the effect of independent variables on the shock decay. The time evolution of all decaying shock waves is shown to be excellently modeled by a single power law, which is used to quantify shock decay properties across all cases. Results indicate that shock unsteadiness is strongly correlated with shock tube sound speed ratio, and its reduction may mitigate the influence of shock unsteadiness on test flows.