## 3 Results and Discussion  
### Bibliometric Analysis of Research on Pavement Management (2000-2013)  
#### 3.1.1 Publication Year, Document Type, and Language  
The distribution of annual publication output identified by Scopus is shown in Figure 1. The publication year of the documents shows the increasing attention (or not) that the subject is gaining in the research community. After the analysis of the figures, three different zones could be identified according to the annual production. The first one, from 2000 to 2002, is characterized by a stable production rate with an average value of 27 publications per year. The next zone, started in 2003 and finished in 2010, shows a quite constant behavior, 51 publications/year, which nearly doubles the initial level. After 2010, there was a continuously increasing general trend in the number of publications that appeared each year. A linear regression was applied to the data, and the result was a very good fit, with an \(R^{2}\) value of 0.987. The assessed slope was 20 publications per year, a figure that gives an idea of the publication rate evolution.  
The distribution of document types was analyzed. It helps in understanding how innovation in this area is transmitted. Eight different document types were found among the 727 publications from the studied time range. Article (379) was the most frequently used document type, comprising 52.1% of the publications, followed by conference paper, with 320 documents (44.0% of the total production).  
Figure 1: Annual publication output.  
The other less significant categories, whose joint contribution was lower than 4%, included review (16), conference review (6), editorial (3), book (1), book chapter (1), and short survey (1). These percentages were not too concordant with the figures obtained by other bibliometric studies that demonstrated the clear supremacy of articles over other types of publication in other engineering areas, such as environmental or chemical engineering [43; 44; 45].  
The singularity of the high importance of the conference papers in this study was more deeply analyzed. The breakdown of the annual production of both main document types is shown in Figure 1. As observed, the contribution of the conference papers was so important that it became the most frequent document type for several years: 2001, 2003, 2005, 2006, 2008, and 2011 (6 years in a 14-year period). This fact could be justified by the celebration, in these years, of some relevant international congresses and conferences, which greatly contributed to attain a high production of conference papers. Table 1 lists the most important congresses and conferences found in the Scopus database. However, not all the important and prestigious congresses or publications in the field of the pavement management, like the International Conference on Managing Pavement Assets, the European Pavement and Asset Management Conferences and the Conferences of the Australian Road Research Boards, are refereed, since they are not indexed in Scopus.  
The language of a publication area reflects the language chosen by researchers to transmit their knowledge. A clear majority of all the publications were published in English (96.1% of the documents). Only six other languages were found, with Chinese appearing as the second language, but with a very low contribution, as the papers written in this language only represented 2.8%. The rest of languages were considered anecdotal: 5 documents in Spanish, 4 in French, and only 1 in Croatian, Korean, and Malay. It is obvious that English has been the main language employed for scientific research since some decades ago, and more specifically, in the engineering field [46; 47; 48]. Additionally, the majority of the bibliometric studies stated that above 95% of scientific publications were written in English [49; 50].