The effects of wood post strength and soil strength on the dynamic performance of guardrail systems have long been a concern of the roadside safety community. Evidence from full-scale crash tests has suggested that these parameters may significantly affect guardrail system performance. Essentially identical tests have resulted in widely varying outcomes that might be the result of various post strengths and soil conditions. A finite-element model of a common guardrail terminal—the modified eccentric loader breakaway cable terminal—was developed to examine the effects of post strength and soil strength on the overall performance of the terminal system. A matrix of 12 simulations of a particular full-scale crash test scenario was conducted with the explicit nonlinear dynamic finite-element software LS-DYNA3D to establish the combinations of post and soil strengths that produce favorable results. The parametric simulations show that certain combinations of soil and post strengths increase the hazardous possibilities of wheel snagging, pocketing, or rail penetration, whereas other combinations produce more favorable results. Conditions that will maximize the safety and reliability of the guardrail terminal system are identified.