## 1 Introduction  
In the near future, vehicles will be connected, becoming mobile sensors of traffic, pollution and driver behaviors. A car today is, in fact, already equipped with thousands of sensors collecting information about the vehicle and its surroundings [1]. When wireless connectivity will also be available, the data collected on board could be forwarded to other vehicles in the surrounding area, road side infrastructures and remote data centers through vehicle-to-everything (V2X) communications, thus enabling new services and applications ranging from safety to traffic efficiency and infotainment. Wireless technologies can, in fact, extend the vehicle's ability to see further down the road by providing 360-degree non line-of-sight awareness. This capability complements the use of sensors, which rely on line-of-sight and typically good weather conditions, limiting their range and capabilities under certain conditions [2]. Hence, the choice of the best wireless technology to enable vehicular connectivity is becoming a crucial issue, discussed both at the round tables of researchers, and at an industrial level; new players are entering the game, not only from the automotive and telco sectors, but also from information technology fields, such as Google, (see, e.g., the Google car), and original equipment manufacturer (OEM), such as Bosch (Stuttgart, Germany) (see, e.g., the artificial intelligence (AI) system on board future Level 3 autonomous cars) and international entities are fighting to propose the wireless access technology that they believe in more. This is justified, for example, by the OEM expected revenues from monetization of automotive sensor data that are expected to reach 706 million US$, representing a compound annual growth rate (CAGR) of 46.8% according to ABI Research a market-foresight advisory firm providing strategic guidance on the most compelling transformative technologies; it foresees that cars contributing sensor data to ingestion platforms is expected to grow to over 60 million by 2023 [3]. About the choice of the enabling technologies, the Car-to-Car Consortium, for example, is leaning toward the IEEE 802.11p and its related standards (such as intelligent transportation systems (ITS)-G5), since it represents the most tested and consolidated technology for vehicular communications and have shown good performance also in challenging scenarios [4]. The 5 GAA is instead promoting C-V2X, which includes both long- and short-range communications based on the cellular standards; in their opinion, short-range C-V2X has the advantage of being included in the same chipset as long-range communications, also promising better performance than IEEE 802.11p due to more advanced solutions at the physical and medium access layers and taking advantage of a more solid industrial ecosystem to guide its future evolution [5]. Besides these two main technologies, other complementary wireless access technologies are under investigation, such as visible light communication (VLC) and mmWave [6], in order to improve security, reliability and/or latency in specific scenarios. The objective of this paper is to provide an insight of the challenges posed by short-range vehicle-to-vehicle (V2V)-based vehicular sensor networks (VSNs) starting from economic motivations up to requirements and related enabling technologies, also presenting an overview of the potential mandatory rules that are being discussed at an international level to integrate connectivity on board of vehicles.