The objective of this paper is to study the influence of the direction of polarization at the transmitter site on the characteristics of the propagation channel in a road tunnel, either empty or in presence of vehicles. The transmitting frequency being on the order of 1 GHz, the tunnel behaves as an oversized waveguide and a simulation based on ray propagation is well suited. To take vehicles into account, the ray-based software models vehicles as parallelepiped considering full polarimetric wave interaction. In the examples described in this paper, the polarization at the transmitter is either vertical, horizontal or making an angle of 45°, referred to the horizontal axis. This last orientation that is often practically encountered in mobile communication systems has, to our knowledge, not yet been studied for a road tunnel which strongly differs from urban or suburban areas due to its guiding effect. Path loss, cross-polarization discrimination factor and characteristics of the polarization ellipse at the receiving point are compared for different configurations.