# Passive RFID-Based Inventory of Traffic Signs on Roads and Urban Environments
## 1 Introduction
However, since the tags are placed on the pavement, not on the traffic signs, this system is not appropriate for the intended inventory purposes. The experimental assessment of [9] confirms that the tags can be only detected at a short distance, achieving a maximum communication range of 40 cm, and the system limits the speed of the cars to 20 Km/h. An improved version of [9], which reduces the number of RFID tags required to work by a more elaborated encoding mechanism, has been presented in [24]. Furthermore, active RFID technology has been proposed \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Work** & **Application** & **Technology** & **Frequency (MHz)** \\ \hline
[9] [PERSON], 2006 & In-vehicle traffic sign detection & Passive & 13.56 \\
[10] [PERSON], 2012 & In-vehicle traffic sign detection & Active & 2400–2483 \\
[11] [PERSON] 2012 & Traffic sign alert system & Active & 867 \\
[12] [PERSON], 2010 & Intelligent speed controller & Active & 433 \\
[13] [PERSON], 2014 & Vehicle positioning in tunnels & Active & 417.05–435.9 \\
[14] [PERSON], 2014 & Vehicle positioning & Active & 902–928 \\
[15] [PERSON], 2016 & Vehicle location for IoT applications & Passive & 0.125–0.134 \\
[16] [PERSON], 2017 & Traffic light recognition & Passive & 13.56 \\
[17] [PERSON], 2011 & Road traffic congestion monitoring & Active & 2400 \\ \hline
[18] [PERSON], 2015 & Traffic congestion control, ambulance & Passive & 0.125 \\ \hline
[19] [PERSON], 2012 & Autonomous vehicle location & Passive & 870 \\
[20] [PERSON], 2015 & Vehicle identification & Passive & 920–923 \\
[21] [PERSON], 2017 & Automatic vehicle identification & Passive & 860–960 \\
[22] [PERSON], 2013 & Recognition of vehicle position & Passive & 917–923.5 \\
[23] [PERSON], 2017 & Vehicle identification & Passive & 910–920 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: RFID systems for road applications. in [10; 11; 12] to increase the transmission range on ADAS systems. Contributions [11; 12] propose the attachment of active RFID tags at the traffic signs as part of high-accuracy vehicle speed-controller systems, which warn the driver of the proximity of speed limit traffic signs and at intersections and other critical points in the city. Specifically in [12], active tags are used to ensure their detection from a distance large enough to permit the operation of a car's speed fuzzy logic controller. The system shows a good performance at low driving speeds (less than 24 Km/h). An additional feature of [12] is the use of two independent RFID readers, which reduce the occurrence of occasional missed tag detections in comparison to the case of employing only one reader. A more exhaustive comparison with [9; 10; 11; 12; 24] will be presented in Section 5, from the obtained experimental results. Active RFID has also been tested in vehicular routing applications as a substitute for GPS when the reception of GPS signals is not possible [13; 14]. In these approaches, RFID tags are arranged in the road or on both sides of a tunnel's edges, and vehicle location techniques based on the received RFID signal strengths are presented. Specifically, GPS errors are more common in dense urban areas, requiring complex positioning algorithms to achieve an effective position triangulation [25]. Finally, the growing interest in addressing the challenges of the internet-of-things (IoT) [15; 26] and infrastructure-to-vehicle (I2V) communication systems [27; 28] brings us to the conclusion that embedding RFID technology in road facilities is likely to become a reality in the near future [28]. Based on the experimental data reported in previous works, such as [10; 11; 12; 13; 14], we can be confident in that active RFID technology ensures a high coverage range even if the reader is placed in a vehicle that runs at high speed, thus fulfilling the potential requirements for a traffic sign inventory system.