## 3 Results
### Research Hotspots, Tendencies, and Orientations
#### 3.2.1 Keywords Analysis  
Analyzing keywords can give us an idea of the hotspots in the research field. The VOSviewer software is used to analyze the keywords of the BIM and digital twins technologies involved in the digitalization of transportation; 1703 keywords were used in the retrieval of 493 papers. After deleting meaningless words, merging synonyms, and \begin{table}
\begin{tabular}{l l l l} \hline
**No.** & **Countries** & **Total Citation** & **Average Paper Citations** \\ \hline
1 & Germany & 708 & 40.81 \\
2 & China & 326 & 2.99 \\
3 & Canada & 241 & 26.85 \\
4 & Italy & 233 & 4.05 \\
5 & Hong Kong, China & 194 & 27.71 \\
6 & United Kingdom & 164 & 9.19 \\
7 & United States & 135 & 5.05 \\
8 & Australia & 134 & 15.42 \\
9 & Pakistan & 117 & 58.5 \\
10 & Egypt & 114 & 57 \\ \hline \end{tabular}
\end{table} 
Table 6: Most Cited Countries and Average Paper Citations in Scopus and WoS. \begin{table}
\begin{tabular}{l l l l} \hline
**No.** & **Affiliation** & **Country** & **Papers** \\ \hline
1 & University of Naples Federico II & Italy & 17 \\
2 & Cornell University & USA & 17 \\
3 & The Hong Kong Polytechnic University & China & 16 \\
4 & Jiangsu University & China & 13 \\
5 & Beijing Jiaotong University & China & 11 \\
6 & Duke University & USA & 11 \\
7 & The University of Hong Kong & China & 11 \\
8 & Chang’an University & China & 10 \\
9 & University of Padova & Italy & 10 \\
10 & Sapienza University of Rome & Germany & 9 \\ \hline \end{tabular}
\end{table} 
Table 7: Most Relevant Affiliations in Scopus and WoS. performing cluster analysis on more than 40 occurrences of keywords, the final results are shown in Figure 6, which presents the collaborations of 100 keywords and the evolution of the top 20 items. In addition to the retrieval of keywords for BIM and digital twins, by integrating the high-frequency keywords in the research on transportation digitization, we can see that there are three phases of the timeline of the development of transportation digitization. As part of the first stage (2015-2017, color purple), the research directions are as follows: Information (2015.4), Buildings (2016.4), Structural Design (2016.5), Surveys (2017.5), Railways (2017.8), Information Theory (2017.8), and Sustainable Development (2017.9). Second comes the development stage (2018-2019, color green), with research directions including Green Building (2018.1), Construction Projects (2018.1), CAD (2018.2), Railroad Transportation (2018.4), Automation (2018.5), Interoperability (2018.6), Traffic Management (2018.8), Computer Simulation (2018.8), Point Clouds (2019.1), Construction (2019.1), Railroads (2019.1), Architecture (2019.1), Numerical Model (2019.2), Energy Efficiency (2019.2), Project Management (2019.2), Optimization (2019.1), Safety Engineering (2019.3), Cost Estimation (2019.4), Intelligent Buildings (2019.4), Model (2019.5), Decision Making (2019.5), Framework (2019.6), Planning (2019.6), Energy Utilization (2019.6), Efficiency (2019.6), Life Cycle (2019.7), Systems (2019.7), Construction Management (2019.7), Maintenance (2019.7), Artificial Intelligence (2019.8), Performance (2019.8), Climate Change (2019.8), Infrastructure (2019.8), Data Visualization (2019.8), Costs (2019.8), Facility Management (2019.8), Bridges (2019.8), and Virtual Reality (2019.9). Currently, related research is in its third phase (2020-2022, color yellow), in which research works on Integration (2019.1), GIS (2019.4), IoT (2019.5), Three-Dimensional Computer Graphics (2019.5), Information Use (2019.7), Visualization (2019.9), Big Data (2019.10), Sustainability (2019.11), Simulation (2019.12), Vehicles (2020.2), Highway Planning (2020.4), Safety (2020.7), Real-Time (2020.9), Data Acquisition (2020.12), Traffic Control (2021.3), Smart City (2021.6), Digital Storage (2021.12), Monitoring (2022.4), Deep Learning (2022.5), and Machine Learning (2022.6) are being widely viewed.