# Survey of Smart Parking Systems
## 2 Methodology Used to Search for and Select the Primary Studies
### Performing the Review
#### 2.2.3 Data Extraction and Synthesis  
Once the steps of the research method were implemented, the result was 274 articles (considered as primary studies) published between January 2012 and December 2019 that either proposed or detailed the development of an SPS or focused on the algorithms used in VRP. Table 4 depicts the steps applied in this work and the results obtained at each stage. As mentioned above, two search methods were used: the automatic and snowballing search methods. These methods were divided into five steps, which were then applied to seek out studies and select a set of primary papers from which to extract the data for this review. * The first step to study selection is the automatic search. This corresponds to applying search strings to the online databases mentioned above to obtain a first set of primary studies of evidence. In this initial step, 1063 papers were obtained when all the results from all of the selected databases \begin{table}
\begin{tabular}{c c} \hline \hline
**Database** & **Search Strings** \\ \hline \multirow{4}{*}{Scopus} & TITLE-ABS-KEY (“Smart Parking System” OR parking OR “car parking”) AND \\  & TITLE-ABS-KEY (“Vehicle Routing Problem” OR “Routing Algorithm” OR “Intelligent Transport System” OR “Artificial Intelligence” OR “Smart Cities” OR “Algorithm” of Planning”) AND (LIMIT-TO (SUBJAREA, “COMP”)) \\ \hline \multirow{4}{*}{IEEE} & ((“Smart Parking System” OR parking OR “car parking”) AND (“Vehicle Routing Problem” OR “Routing Algorithm” OR “Intelligent Transport System” OR “Artificial Intelligence” OR “Smart Cities” OR “Algorithm of Planning”) \\ \hline \multirow{4}{*}{ACM Digital Library} & (“Smart Parking System” + parking + “car parking”) AND (+“_Vehicle Routing Problem” + “Routing Algorithm” + “Intelligent Transport System” + “_Artificial Intelligence”_ + “Smart Cities” + “Algorithm of Planning”) \\ \hline \multirow{4}{*}{TRID} & (“Smart Parking System” OR parking OR “car parking”) AND (“Vehicle Routing Problem” OR “Routing Algorithm” OR “Intelligent Transport System” OR “Artificial Intelligence” OR “Smart Cities” OR “Algorithms of Planning”) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Search strings for online databases. were totaled. Over this total, 350 works were obtained from Scopus, 340 from IEEE, 276 from ACM Digital Library, and 97 from TRID. * The second step is analysis of titles and duplicates papers. The previously defined inclusion/exclusion criteria, in Section 2.2.1, were applied to the set of papers obtained in the first step and took into account the title of each article. In addition, the duplicate articles obtained from the different online databases were eliminated. For this step, 611 articles were selected. * The third step is the meta-data analysis. Meta-data refers to the abstract and keywords in each article and determines whether these comply or not with the selection criteria. The most relevant work, for this study, after applying this step, amounted to 388 articles. * The fourth step is full-text analysis. In this step, the full text from the papers obtained is analyzed in order to produce a more thorough examination of their compliance with the selection criteria. One this is done, those that meet the inclusion criteria are chosen. Once this step was completed, a total of 253 papers comprised the final set of papers. * The fifth step is the snowballing search technique. This final step consists of applying the selection criteria to the studies found using the first search method. The snowballing search technique is used to select those papers that may have escaped the automatic search, thereby allowing us to find all possible evidence. This method consists of reading the list of references or quotations (backward snowballing) for each article from the set of papers, and analyzing the citations given for these articles (forward snowballing), to search for other sources or primary papers.