# Passive RFID-Based Inventory of Traffic Signs on Roads and Urban Environments
## 2 System Description and Implementation Aspects
### Implementation Aspects
#### 2.2.3 Costs Considerations  
Costs considerations lead us to propose a system based on passive RFID technology, as described in Section 1. In passive RFID, tags collect the energy required for operation from the radio-frequency signal received from the reader device. By contrast, active RFID tags have a local power source (e.g.,  
\begin{table}
\begin{tabular}{l c l} \hline \hline
**Field (Used Name)** & **Length (bits)** & **Description** \\ \hline Signal (Sg) & 10 & Indicates the type of signal. \\ \hline Signal number (SN) & 10 1 & Identifier of the traffic sign within its stretch. \\ \hline Stretch (Sd) & 8 2 & Identifies the stretch within a road. \\ \hline Road (Rd) & 26 3 & Road identifier. \\ \hline Kilometer (Km) & 11 4 & Indicates the kilometric location on the road of the traffic sign. \\ \hline Meter (M) & 10 & Indicates the distance in meters from the start of the kilometer to the traffic sign. \\ \hline Installation date (InD) & 10 5 & Indicates the date when the traffic sign was installed, in order to know if it has to be replaced. \\ \hline Cyclic redundancy check (CRC) & 5 & Code to detect possible errors in the reception of the EPC. \\ \hline \hline \end{tabular} 1 This length will be oversized, since it allows to code around five traffic signs per meter. 2 This length is 8 bits because the longest road in Spain (named N-340) is 1248 Km long, so it is composed of 250 stretches. 3 This length is 26 bits because the usual format name for a Spanish road is composed of two letters and a three-digit number, requiring 16 bits for the letters (using an ASCII 8-bit code) and 10 bits for the number. 4 This length is 11 bits because the maximum length is 1248 Km. 5 This length is 10 bits in order to use 4 bits to codify the month and 6 bits to codify the year.  
\end{table}
Table 2: Codification of the EPC frame.  
a battery) so that they can transmit data farther than passive tags. However, the cost per active RFID tag (around US$20-25 in the time of writing) is also much higher than that in the passive case (about US$3 for a ruggedized passive tag designed to work over metallic surfaces) [35; 36]. Moreover, considering a price of around US$100-350 [37] per traffic sign, the total cost should be considerably increased installing active tags in comparison with using passive technologies. This difference regarding costs is extremely important, because the final objective of this research is to implement an inventory system for all the traffic signs which compose the Spanish National Roads Network. Therefore, placing a passive tag at each traffic sign, instead of an active tag, would be saving several million US$.  
Besides the costs per tag, active technology requires batteries, which must to be replaced periodically, leading to an additional increasing maintenance costs. Additionally, active systems commonly employ proprietary communication protocols, which imply a manufacturer dependency with respect to future improvements of the system. At this point, these factors clearly tip the balance in favor of passive tags. Therefore, the following sections will describe the validation of the selected technology at theoretical, simulation, and experimental levels.