## 1 Introduction  
Due to the growing number of vehicles on the roads worldwide (more than two billion by 2050 [1]), road traffic accidents are currently recognized as a major societal and public health problem. According to the World Health Organization (WHO) [2], the total number of road traffic deaths due to accidents remains unacceptably high at around 1.24 million per year, in addition to twenty to fifty million who are injured and/or disabled. Projected trends suggest that road traffic injuries will become the fifth main cause of deaths by 2030. Road accidents and crashes have a major impact on national economies. For instance, the Automobile Association of America (AAA) estimated that the total cost of traffic crashes is around 166.7 billion USD [3]. This includes the costs of medical, emergency and police services, property damage and quality of life. Similar estimates from the WHO show that traffic injuries cost middle-income countries around 2% of their gross national product (over 100 billion USD per year). The above projected trends will have a tremendous impact on our society and our quality of life and will ask for specific actions to be put in place to deal with these in the next coming years. More specifically, traffic accidents can be partly mitigated through the launch of specific awareness campaigns and national strategic programs to promote a culture of safe road behavior (e.g., reducing speed, systemic usage of seat belts, _etc._), to enforce traffic laws (e.g., warning, reporting, summonsing, _etc._) and to plan for the design of safer roads and transportation systems. In this context, cooperative intelligent transport systems (ITS) are considered as the key enabling technology to improve road safety, traffic efficiency and driving experience. There is currently a trend to deploy this new technology in vehicles to enable them to communicate not only with hotspots, but also with other nearby entities through direct short-range communications (DSRC), such as vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), vehicle-to-pedestrian (V2P) and vehicle-to-anything (V2X). Indeed, new market forecasts [4] clearly indicate that the connected vehicle market will be worth 39 billion USD in 2018, up from 13 billion USD in 2012, and that more than 50% of the vehicles sold worldwide in 2015 will have communication capabilities. It is envisioned that every new vehicle will be connected in multiple ways by 2025. This new reality of intelligent transport systems will leverage different types of vehicular communications (VC) and telematics services to enable the emergence of innovative active road safety applications, driver assistive services, smart mobility and traffic management services. Research and standardization activities on intelligent transport systems had significantly started, worldwide, more than a decade ago [5,6] and encompasses various multidisciplinary areas, including radio channel modeling, data link protocols, wireless communications, networking protocols, security and localization. Standardization of ITS is also important to ensure interoperable V2X communications between different equipments and systems, regardless of their brands and models. In the United States,IEEE proposed a novel Wireless Access in Vehicular Environments (WAVE) V2X protocol stack [7] for enabling future ITS applications. It relies on the IEEE 802.11p standard [8], which extended the PHY and MAC layers of the IEEE 802.11-2007 standard [9] to cover vehicular environments and the IEEE 1609 family of standards [7] to provide high level features (above the MAC layer), such as routing, addressing, security and resources management. In Europe, the European Telecommunications Standards Institute (ETSI) TC ITS working group [10] is working on a global standard for cooperative ITS systems, which adapts and optimizes ongoing ITS proposals at IEEE and ISO [11]. The ETSI TC ITS defines a reference architecture [10] for cooperative V2X communications, including the support for the IEEE 802.11p standard [8].