# Usage of V2X Applications in Road Tunnels
## 1 Introduction  
Today, the importance of city management is focused on new technologies, sustainability, social aspects, and complex solutions. It is based on the realization of new cases, architecture improvements, IoT deployment, data connectivity, and integration of these particularities, m main objective of ensuring acceleration, the knowledge-based approach, future strategies, and sustainable evolution of livable cities [1, 2, 3]. The importance of livability also lies in the movement, accessibility, social, and environmental aspects that fall within the scope of urban transportation [4]. The hidden but essential parts of the transport infrastructure are the tunnels that ensure smooth traffic and connections between the city districts. Tunnels seem to be the perfect way to achieve the criteria for a smart city mentioned above, but several drawbacks are associated with them, especially from a technological point of view. This hidden part of the infrastructure does not allow common positioning systems [5], their applications, and sometimes data connectivity, and thus the tunnels are smart and also ineffective for many applications at the same time. In the context of the article, the meaning of infrastructure is related to the tunnel itself (construction), control systems, equipment, and devices implemented in the tunnel, as well as in the surroundings, such as traffic light controllers, variable message signs, informative panels, etc., which are implemented on the access ramps to the tunnel and the tunnel exits. Tunnels are an essential part of urban engineering and smart city ecosystems. First, due to the impact on traffic in the city [6], and second, due to the wide range of installedequipment that provides the possibility of data acquisition, to regulate the maximum allowed speed or entrance into the tunnel, and thus indirectly the number of vehicles in the city districts [7]. The implementation of the tunnel complex in cities brings many benefits, but the financial aspect of investing in construction and then even more in operation and maintenance is not negligible [8; 9; 10]. One of the major problems concerning tunnels is the Global Navigation Satellite System (GNSS) availability [11]. Since the tunnel is \"hidden\", many applications are limited due to positioning requirements. Some of them only notify the user that the GNSS signal is lost, while other ones are based on statistical methods, computation, usage of external devices for supplementary measurements, communication, etc. [12; 13]. The problem of a lack of GNSS signal availability has been occurring in tunnels for many years. Whereas smart technologies, navigation systems, and other applications are growing in popularity together with the trends of smart, resilient, and sustainable cities, the problem with GNSS is also growing and its importance is increasing. For this reason, the problem becomes the issue of how to solve positioning (not only) in tunnels. For the mentioned reasons, it is necessary to take into account these blind spots in smart mobility and transportation solutions. Tunneling and tunnel equipment should be one of the main disciplines in urban planning and engineering. It is important to provide a solution for all road users, including road users in tunnels, where, for example, navigation is crucial, and its functionality may have implications on traffic throughout the city. However, the most important to ensure are the safety applications, e.g., smooth passing of an emergency vehicle, dissemination of warning information, as well as about the failure in tunnels or tunnel closure. Vehicular communication (V2X--vehicle-to-everything) is one of them [14; 15], but it does not work properly in locations where GNSS is not available, and thus the analysis of V2X communication in the tunnel is presented in the article with a focus on assisted positioning methods due to V2X communication requirements in position accuracy and its continuous updating during the data transmission.