## 1 Introduction  
In the last years, the interest in vehicular communications has rapidly increased. The main reason behind this is the fact that the establishment of ad hoc communication networks between vehicles, as well as with the corresponding infrastructure--referred to as vehicle-to-everything (V2X) communication in the literature--is expected to considerably reduce the number of accidents and to improve traffic management. The benefits of this paradigm, which is called the Intelligent Transportation Systems (ITS) in the literature, are so important that research in this area is considered as a strategic topic for transport authorities in the European Union and most developed countries. When it comes to V2X communication, there are currently two competing technologies, i.e., Dedicated Short-Range Communications (DSRC) and cellular-V2X (C-V2X). The former is based on the IEEE 802.11-2012 standard, which was later superseded by the IEEE 802.11-2016 standard. On the other hand, the C-V2X was described in the specifications of 4G LTE Release 14. The DSRC technology was mainly developed to enable collision prevention applications [1]. It is widely regarded to be reliable, adequate, and, most importantly, immediately available. Moreover, it is also almost patent-free, rather easy to implement, and financially rather cheap. On the other hand, the C-V2X is claimed to have a wider application range in areas such as entertainment, traffic data, navigation, and, most notably, autonomous driving. In comparison to the DSRC, it can leverage existing infrastructure and can cover larger areas with less infrastructural equipment, thus possibly reducing the capital and operational expenditures for the infrastructure's owners [2], but unfortunately not for its users [3], unless a no-SIM operational approach is adopted. Some work has been carried out to separately evaluate the performance of C-V2X--i.e., LTE-V2X--and IEEE 802.11p, as well as on a comparative basis. In [4], intensive data analytics on V2V performance and characterization of the V2V channel based on real-world urban DSRC traces were presented, and a reliable context-aware beaconing strategy called CoBe was proposed to enhance the broadcast reliability by coping with harsh NLoS conditions. Moreover, the authors of [3] dealt with the application-level performance of safety-related communications over IEEE 802.11p in the Infrastructure-to-Vehicle (I2V) context and presented the first outcomes resulting from several short- and medium-range field tests. They studied five different performance metrics that were relevant at the network, transport, and application levels. Their figures and trends were examined against six variables, i.e., distance, transmission power, antenna quality, congestion, interference, and speed. On the other hand, the performance of LTE-V2X in the context of the I2V and V2V communication was investigated by using simulations in [5]. When it comes to the benchmarking of C-V2X and IEEE 802.11p in terms of their performance, the following studies focused on this issue: [6; 7; 8; 9; 10; 11; 12]. In [6], the authors benchmarked LTE-V2X and IEEE 802.11p in terms of the average packet reception ratio in typical urban and freeway scenarios and the V2V context. The DSRC and LTE-V2X were compared from the perspective of the packet delivery success rate in different transport scenarios, i.e., cities and highways, in [7]; again, this was only in the V2V context. In [8], ITS-G5 and LTE-V2X (mode 3) were benchmarked in terms of the end-to-end delay and radio frequency conditions by measuring the signal to interference plus noise ratio and reference signal received power in two scenarios, i.e., the effects of realistic data traffic on an ITS alert service and the impact of handover on an ITS safety service; again, this was only in the V2V context. In [9], the benchmarking was performed in an urban micro-cell highway scenario and the V2V context from the perspective of the packet reception ratio and transmitter-receiver distance.