In the actual traffic environment, there usually exists more than one vehicle in the radiation area of the road-side units (RSU) at the same time, and the front vehicle will affect the ray paths between the RSU and the on-board units (OBU) of the back vehicle. Here we studied the front vehicle’s impact on the ETC system’s path propagation loss by ray tracing technology and Uniform Theory of Diffraction (UTD). Firstly, we simplified the vehicle body structure into two equivalent geometric models. We analyzed propagation loss models under line-of-sight (LOS) and non-line-of-sight (NLOS) conditions based on the distance change among RSU, OBU, and the front vehicle. Finally, a set of ETC comprehensive testing equipment was developed to measure the transmission loss of toll stations. Both simulation and experiment results indicate that the propagation loss models proposed in this letter are valid.