# Road Infrastructure Challenges Faced by Automated Driving: A Review
## 3 Limitations and Advances of Road Infrastructure
### Traffic Sign Recognition System (TSRS)
#### 3.1.2 Advances  
How technology copes with limitations introduced in the previous section can be viewed from two perspectives: vehicle and road. One way to increase the reliability of TSRS from the vehicle perspective is to introduce hardware and software redundancy by integrating LiDAR together with the camera. LiDAR is capable of precisely defining the position of 3D objects and includes their reflectivity information, which adds another layer to detection and recognition [57]. Therefore, sensor fusion has great potential for increasing the accuracy in sign interpretation. From the road perspective, using a higher grade of retroreflective material/sheeting on traffic signs improves overall visibility under all environmental conditions and increases robustness regarding sign degradation over time [55; 58]. Improving traffic signs or sensor systems by itself, however, will not fully eliminate the previously mentioned limitations regarding inclement weather or damaged traffic signs. Therefore, to link those two worlds, V2N and V2I solutions are needed. The majority of speed assistance systems that rely on TSRS, among others, are however integrated with the spatial mapping system with satellite position and databases of speed zones [11]. Yet, further development of digital maps, smart sign technology and short-range communication has great potential to introduce a redundant system for both road infrastructure and MV and allow the system to work under different environmental conditions and cope with signs that have a distorted appearance.  
In the last decade, other technologies have also been applied to traffic signs. 3M introduced the \"invisible\" 2D barcoded road sign. The idea is to place the IR spectral barcode on the traffic sign, which makes it invisible to humans but readable by ADAS up to a distance of approximately 90 m [59]. RFID technology is another case study. It offers a short-range solution, up to approximately 30 m, for localisation and maintenance tasks [60; 61; 62]. Table 6 summarises the advances in technology regarding TSRS.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{3}{c}{**Vehicle and Sensor Perspective**} \\ \hline \(\bullet\) & Integration of LiDAR and cameras brings redundancy and enhances current TSRS with precise localisation of 3D geometric features [57]. \\ \hline \multicolumn{3}{c}{**Road Infrastructure Perspective**} \\ \hline \(\bullet\) & Higher grade retroreflective material (sheeting) improves overall visibility under all environmental conditions and increases robustness regarding sign degradation over time [55]. \\ \(\bullet\) & IR spectral barcodes provide human-invisible technology, a potential technology for the transition period [59]. However, more expensive IR cameras are needed. \\ \(\bullet\) & RFID technology offers a short-range alternative for localisation and updating traffic sign databases [60; 61; 62]. \\ \hline \multicolumn{3}{c}{**Digitalisation Perspective**} \\ \hline \(\bullet\) & Digital maps and short-range communication provide a link between vehicle and road while introducing additional redundancy and robustness regarding differences in environmental conditions and the appearance of traffic signs. \\ \hline \hline \end{tabular}
\end{table}
Table 6: Advances in traffic sign recognition-based systems.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{3}{c}{**Design**} \\ \hline \(\bullet\) & Collocated traffic signs that apply to different motorists or are time-dependent and weather-based [11]. \\ \(\bullet\) & Flickering of electronic signs [11]. \\ \(\bullet\) & TSRS systems cannot currently interpret text qualifications [11]. \\ \(\bullet\) & The similarity in shape between the numerals (e.g., 30, 60 and 80 km/h) [11]. \\ \(\bullet\) & The similarity in shape and colours [11]. \\ \(\bullet\) & Signs not installed by traffic authorities: signs printed on rubbish bins, heavy vehicles [11]. \\ \hline \hline \end{tabular}
\end{table}
Table 5: _Cont._