The crash barrier should not only have enough height to block the vehicle from crossing but also prevents the vehicle from tilting or passing through. In the past, we thought the height of the guardrail was the height from top to the ground. In the beam column guardrail system and the combined guardrail system, the resistance R of the guardrail is usually not located at the top of the guardrail, but slightly lower. The effective crash barrier height is the height of the guardrail resistance R away from the ground. Therefore, considering the influence of vehicles tilting by guardrail’s height, the effective height of the guardrail is more important than the total height of the guardrail. The United States discovered that many guardrails were crossing through by vehicles from the barrier crash accident. It is not due to the strength of guardrail, but the insufficient effective height.