## 4 Bibliographic Analysis
The study on the correlation encountered a good relationship in the research carried out from 1996 to 2018, which confirms the close relationship between the different organizations (\(r^{2}\) = 0.869 and \(p<<0.05\) for \"Universities\" versus \"Institutions\", \(r^{2}\) = 0.717 and \(p<<0.05\) for \"Universities\" versus \"Corporations\", and \(r^{2}\) = 0.691 and \(p<<0.05\) for \"Institutions\" versus \"Corporations\"). The study also encountered that the universities started the research in this field three years earlier than the other organizations (Figure 6). This, in addition to the greatest number of publications, suggests that universities mainly carry the weight of the investigation in this field. This can be derived from the most productive organizations resumed in Tables 2 and 3, where the top of universities, institutions, and corporations is shown. An analysis on the source type of the publications (Figure 7) shows a major contribution through conferences (62.41%) followed by journals (30.96%) and books (6.62%). The study confirms a good correlation between the different sources, which suggests that a rapid and strong growth in conferences is also reflected in scientific journals and textbooks (\(r^{2}\) = 0.849 and \(p<<0.05\) for \"Conference Proceedings\" versus \"Journals\", \(r^{2}\) = 0.887 and \(p<<0.05\) for \"Conference Proceedings\" versus \"Book Series\", and \(r^{2}\) = 0.891 and \(p<<0.05\) for \"Book Series\" versus \"Journals\"). The study on the document type found a total of 219 open access manuscripts (6.37%) as well as 41 surveys and short reviews (1.19%) over a total of 3439 manuscripts. These results reflect the weight of primary research versus secondary research. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**R** & **Country** & **MP** & **MP (\%)** & **CA** & **IC** & **CR** & **CR (\%)** \\ \hline
1 & USA & 667 & 19.24 & 344 & 46 & 7856 & 31.55 \\
2 & China & 538 & 15.52 & 340 & 30 & 3573 & 14.35 \\
3 & S. Korea & 332 & 9.57 & 101 & 22 & 1266 & 5.08 \\
4 & Germany & 302 & 8.71 & 175 & 32 & 3600 & 14.46 \\
5 & France & 221 & 6.37 & 167 & 36 & 1690 & 6.78 \\
6 & India & 219 & 6.31 & 26 & 14 & 676 & 2.71 \\
7 & Japan & 207 & 5.97 & 72 & 19 & 919 & 3.69 \\
8 & Spain & 178 & 5.13 & 133 & 29 & 2337 & 9.38 \\
9 & Canada & 139 & 4.01 & 109 & 29 & 1465 & 5.88 \\
10 & UK & 133 & 3.83 & 131 & 39 & 1706 & 6.85 \\ \hline \end{tabular} R: Ranking position; MP: Manuscripts published; CA: Co-authorships with other researchers; IC: International collaborations; CR: Citations received.  
\end{table} 
Table 1: Most productive countries in I2X, V2X, and P2X research from 1996 to 2018. Figure 6: Bibliographic evolution of the contributions on I2X, V2X and P2X research per organization. An in-depth review on the co-occurrence of the keywords between the publications--both indexed by authors and publishers--is shown in Figure 8. The network visualization corresponds to the 1000 most popular terms over a total of 14,364 keywords within the 3422 manuscripts. Accordingly, the number of documents in which some representative keywords related to this survey appear resulted as follows: \"V2V\" (2476), \"V2I\" (896), \"VANET\" (872), \"intelligent systems\" (669), \"V2X\" (286), \"IEEE 802.11p\" (191), \"LTE\" (116), \"MANET\" (112), \"WAVE\" (103), \"5G\" (98), \"VLC\" (83), \"IoT\" (79), \"sensors\" (52), \"D2D communication\" (52), \"WSN\" (37), \"I2V\" (37), \"IEEE 802.11s\" (27), \"smart city\" (27), \"mobile devices\" (21), \"smartphone\" (21), \"RFID\" (21), \"3G\" (20), \"WiMAX\" (19), \"Bluetooth\"