# Passive RFID-Based Inventory of Traffic Signs on Roads and Urban Environments
## 1 Introduction
Nevertheless, the advantages of using a RFID-based architecture would be even more pronounced if passive, not active, RFID were used, as maintenance costs would be greatly reduced in that case (passive RFID does not require battery replacement) and the price per passive tag is insignificant compared to the traffic sign's price. A more exhaustive comparison regarding costs will be detailed in Section 2.2.3. Although there are previous works based on passive RFID for road applications [15; 16; 18; 19; 20; 21; 22; 23], they are mainly focused on vehicle location [15; 19; 22] or vehicle identification [20; 21; 23] purposes. Vehicle location systems require placing multiple tags on the floor to accurately locate the vehicle passing over them, and vehicle identification systems require placing the reader just on the floor to identify the tag attached to the vehicle passing over the antenna. Thus, the required distance range for both applications is lower than that required for a traffic sign inventory application. Some features of these approaches will be discussed in Section 5. Additionally, other passive applications working at different frequency bands, such as [16; 18], present low distance ranges (<10 cm) as well. Therefore, some doubts may arise about the adequacy of passive RFID for the intended tasks. Its lower coverage range, which leads to short tag detection times, more noticeable when the vehicle's speed increases, may prevent the traffic signs from being identified during the normal operation of the system--even considering the recent improvements on the aforementioned limited read range of passive RFID tags [29] or on their performance on metallic environments [30; 31]. As this problem has not been studied in the previous RFID-based approaches for inventory and vehicular routing applications, the aim of the present paper is to fill this gap and validate the use of passive technologies for the inventory purposes claimed. To this end, a theoretical model of the wireless communication link for in-motion scenarios is presented in this paper, and validated at both simulation and experimental levels. A preliminary version of this research was conducted in [32], where some of the functionalities and performance of the proposed system were presented. Besides the theoretical model and its validation mentioned above, this paper presents additional contributions with respect to [32], such as an experimental comparison between passive and active RFID, and the addition of location capabilities, by taking advantage of the information that can be codified in the tags. The integration of both capabilities leads to an additional improvement over several previously referenced approaches, which only provide a solution for inventory [9; 10; 24] or location [13; 14] applications. The paper is organized as follows: Section 2 presents a description of the proposed system and details its implementation aspects, such as RFID tag requirements, codification of their identifiers, employed software, and cost considerations. Section 3 presents a model of the wireless RFID link, which is used to study the reliability of RFID passive technologies for this application. In Section 4, the proposed system is validated at simulation level by using the model developed in the previous section, and at experimental level by describing a comparison between passive and active technologies, and validating the final on-board integrated system. In Section 5, the results are discussed from a comparison with other related works. Finally, Section 6 brings the paper to a conclusion.