## 2 Development of Monitoring Technology for Bridge Expansion Joint Using Line-Scan Cameras  
Recently, the Korean government has been actively encouraging the introduction of the latest structural safety inspection technologies through the revision of various laws and regulations. For example, Annex 10 of the Enforcement Decree of the Special Act on the Safety and Maintenance of Facilities was amended in March 2020 to newly establish a provision for \"appearance investigation and image analysis using new technology or inspection robot, etc.\". In other words, new technology can be applied for structure inspection. Further, the revision of Article 167 of the Occupational Safety and Health Act emphasized the safety management of internal employees by strengthening the punishment standards for employers (a maximum fine of $100,000 USD) in case of workplace accidents caused by negligence in worker safety management [1, 2]. As a result, the structure management conditions of the Korea Expressway Corporation are becoming increasingly strict. The establishment of facility and performance evaluation following the revision of the Special Act on Safety and Maintenance of Facilities newly necessitated the performance evaluation of 2611 Type 3 facilities (bridges) and 4626 structures. As a result, structural inspectors performed 25,219 regular safety inspections, 2205 precise safety inspections, and 258 precise safety diagnoses in 2020, including in-house and external services. A shortage of management personnel is arising owing to this excessive workload (see Table 3). Given that the stricter structure management conditions are imposing greater demands on manpower, it is crucial to promote inspection methods and smart technologies that can replace manpower. The Korea Highway Corporation Road Traffic Research Institute analyzed past safety inspection and technical advice data and found that 78.3% of expansion joint devices that had to be replaced lacked a gap. Since 2016, we have been researching monitoring methods to preemptively maintain a suitable gap between expansion joints [2]. For this purpose, by integrating high-speed line-scan cameras, which are widely used in various fields such as semiconductor inspection and road pavement investigation, image-processing technology to determine the length of expansion and contraction through AI-based (i.e., machine learning) image sensing in the analysis process, and automatic control technology, we developed the **N**onstop bridge **EX**pansion joint gap measuring **U**tility **S**ystem (NEXUS) to measure the distance between expansion joints with 1.0 mm resolution while driving at a high speed of 100 km/h [1] (see Figure 2). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Safety Management** & **Sum** & **Bridges** & **Tunnels** & **Box Culverts** \\ \hline Sum (EA) & 27,682 & 15,636 & 2118 & 9928 \\ \hline Regular safety inspection 1 & 25,219 & 13,648 & 1643 & 9928 \\ \hline Precision safety inspection 2 & 2205 & 1783 & 422 & - \\ \hline Precision safety diagnosis & 258 & 205 & 53 & - \\ \hline \hline \end{tabular} 1 Regular safety inspection: an average of approximately 450 EA/yr/branches performed by the structural staff. 2 Precision safety inspection (2020): self-manpower performance (684 EA), performing external services (1521 EA).  
\end{table} 
Table 3: Number of structural inspections (2020). Figure 2: Development system (NEXUS) and main function configuration. Existing methods manually measure the gap in highway bridge expansion joints through traffic control and partial blockage. By contrast, the proposed method uses a high-speed line-scan camera mounted on a vehicle driving at 100 km/h to acquire an image across a 40 cm-wide car lane. This system uses the geographical information system data of the bridge (above, longitude coordinate system) to perform automatic measurements in conjunction with the mounted GPS while driving at high speed and thereby creates a database of the expansion gaps based on accurate survey images without affecting the traffic flow.