Citation: Brož, J.; Tichý, T.; Prokeš, R.; Štencek, A.; Šmerda, T. Proximity Approach to Bluetooth Low Energy-Based Localization in Tunnels. Sustainability 2023, 15, 3659. https://doi.org/10.3390/su15043659 Academic Editor: TamásBányai Received: 10 January 2023 Revised: 11 February 2023 Accepted: 13 February 2023 Published: 16 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article Proximity Approach to Bluetooth Low Energy-Based Localization in Tunnels Jiˇ rí Brož 1, * , Tomáš Tichý 1 , Radovan Prokeš 2 , Adam Štencek 2 and Tomáš Šmerda 1 1 Faculty of Transportation Sciences, Czech Technical University, 110 00 Prague, Czech Republic 2 CEDA Maps a.s., 140 00 Prague, Czech Republic * Correspondence: jiri.broz@cvut.cz Abstract: Smart cities and smart mobility necessitate the development of newly adapted approaches and applications that ensure data connection among their particular parts; thus, the complex systems of smart technologies can be applied. Especially in the field of transport, mobility, and smart solutions, many of these applications require precise localization. Global navigation satellite system (GNSS) localization is used mainly because of its availability, reliability, and sufficient precision. However, there are also blind spots in smart cities where the GNSS is not available, e.g., in tunnels. This article presents an approach to fixing the issue of localization in road tunnels based on telecommunications with Bluetooth low energy (BLE) based infrastructure equipment. This approach enables us to ensure sufficient data resources for localization, and thus, smart applications can also be applied in the tunnels. At the same time, this approach also allows the fulfillment of individual needs according to the specifications of each tunnel or smart application. The outputs from testing and measurements are presented and discussed. Keywords: BLE; GNSS; localization; proximity; RSSI; smart solutions; tunnels 1. Introduction Localization is a key component of advanced applications and vehicle systems as well as precision. It is essential for navigation services, route planning, and other services that lead to intelligent mobility. Cooperative systems are an example that requires position- ing accuracy to provide data transition [1], but this is equally important for the field of autonomous vehicles and other smart applications. Most of the services provided use the global navigation satellite system (GNSS), and this approach covers a significant part of the road network. However, there are places where GNSS-based localization is inaccurate or unavailable. This may be, for example, in areas of dense and high-rise buildings where signal is limited. In such cases, inaccuracies may occur. There are methods where, on the contrary, by using GNSS signal reflections in conjunction with 3D city models [2], the localization method is refined not only based on triangulation, but also on the basis of signal strength. However, in terms of traffic, there are also places where GNSS-based services are not even theoretically possible to use, e.g., mountain passes, in particular underground structures such as tunnels that are not covered [3]. This type of road infrastructure is very important from a traffic point of view due to the high volume of traffic flow, the lack of an alternative route, and also due to additional traffic-related risks (changing environmental conditions, brightness, or temperature) [4]. The use of precise location and traffic data exchange is necessary for emergency services or tunnel operators, who need to know the exact positions of vehicles. Knowledge is also needed for drivers, especially if there is a crossing, a complex structure of entrances, or exits from the tunnel itself, but mainly it can reduce the mentioned traffic related risks. There is also a need for accurate navigation for future autonomous vehicles. Sustainability 2023, 15, 3659. https://doi.org/10.3390/su15043659 https://www.mdpi.com/journal/sustainability