Pre-collision system (PCS) that basically consists of warning buzzer, brake-assist and automatic brake functions is designed to help mitigate injury occurring in frontal impacts where preceding vehicles are impacted from the rear. Since the benefit of each function is influenced by drivers' reaction before collision, it is difficult to estimate quantitative effectiveness for improvement. This paper proposes an approach to establish a Safety Impact Methodology (SIM) for the effectiveness estimation of a PCS by utilizing driving simulator (DS) test results and Event Data Recorder (EDR) data. The estimation procedure consists of four steps. Firstly, the PCS functions were modeled as linear transfer functions with gain, delay, etc. Test vehicle's properties were measured and modeled as an example. Secondly, the transfer functions were applied to each DS test result, and the speed reduction from travel speed to impact speed was calculated by the model assuming that PCS was installed. Thirdly, considering the activation timings of the PCS functions, how much PCS could work was identified using the EDR data and the DS test result. Finally, the expected speed reduction and thus estimated effectiveness by PCS was obtained by comparing the speed reduction between with/without PCS-equipped cases. By changing the model parameters, the method enables an examination of the influence of the system parameters on improvement quantitatively.