In this paper we present a series of experiments in order to gain insight about the performance of ITS-G5A V2V communications in critical scenarios for autonomous driving. Critical tasks such T-intersection managing in semi-urban environments or elevation changes in two ways inter-urban roads have been identified as challenging scenarios in which traditional sensor based approaches may fail. For this purpose, we designed a set of experimental tests in real environments with automated vehicles equipped with GPS, ITS-G5 compliant V2V communications, cameras and radars. Cameras and radars range is compared to that of the V2V communications in the designed critical scenarios and conclusions are drown. Packets Delivery Ratios (PDR) and the Complementary Cummulative Distribution Function (CCDF) of the Update Delay (UD) are used as metrics to evaluate the quality of the communications and to analyse the requirements of the possible automated driving applications. The obtained results show that ITS-G5 V2V communications offer better performance than on-board sensors in all cases, only being affected by occlusions with big obstacles such as buses or trucks.