## 2 Systematic Literature Review Methodology and Analysis  
A literature review for LML was performed based on a systematic literature review procedure widely adopted in other domains [10; 21]. First, the objective and scope of the literature review on LML were established. In this study, the main objective was to provide a systematic review of recent LML-related studies to investigate the definition and typology of LML, main issues and concerns in LML, and new opportunities for the next generation of LML. Based on a preliminary search of highly cited LML studies, significant keywords that include \"last mile logistics\", \"urban logistics\", \"city logistics\", \"typology of last mile logistics\", \"last mile logistics innovation\", \"collaborative last mile logistics\", \"cooperative last mile logistics\", \"sustainable last mile logistics\", and \"robust last mile logistics\" were identified to better search target research articles. Different combinations of these keywords using Boolean AND and OR operators were used to retrieve initial articles published between 2001 and the end of August 2021 in scientific platforms--EBSCO, Elsevier, Google Scholar, JSTOR, Science Direct, Springer, Web of Science, and Wiley Online Library. Examples of the keyword combinations for the literature search are (\"last mile\" AND \"logistics\"), (\"last mile logistics\" AND \"urban logistics\"), (\"last mile logistics\" AND city logistics\"), (\"last mile logistics\" AND \"innovation\"), and (\"collaborative last mile logistics\" OR \"cooperative last mile logistics\"). Various research works including these keywords or their combination in the title, keyword list, or abstract were disclosed. Consequently, more than 400 research works written in English were collected as the initial article set for review. Next, the following search criteria were applied to refine the initial article set: (i) inclusion of research areas related to LML, and (ii) inclusion of peer-reviewed journal articles having meaningful keywords in their abstract. Then, 169 research articles from 84 unique journals were identified as LML studies that contributed to recent LML advances, but 15 journals were associated with more than three articles each (Figure 1). This indicates that a small number of journals dominate LML studies over most other journals. Finally, the 96 research articles which had more than 10 citations were selected as target articles. The literature demonstrates that LML is an emerging research area with rapid growth (Figure 2a). The black solid line and the green dashed line represent the total number of and the average number of publications, respectively, while the red line means the number of publications for each year. Moreover, Figure 2b shows the top eight countries producing the most LML research based on the affiliation of the first author. Furthermore, the top five articles in terms of the number of citations are [22] (476), Ref. [23] (331), Ref. [24] (243), Ref. [25] (236), and [26] (233). An overview of the literature review method used for this study is presented in Figure 3.  
To better understand the underlying themes addressed in the target research article set, text mining analysis was performed using the following procedure. First, text data were collected from the title, abstract, and keywords of each article and separately saved to a text file. The set of text data was processed by the \(tm\) package for the \(R\) statistical program language [27] to standardize the text data; stop-words, whitespaces, numbers, and punctuation were removed from the original text dataset, and all letters were transformed to lower case. In addition, each word in the set of text data was stemmed by the _Snowball_C package [28] to regard word variants originated from the same root as the same term  
Figure 1: Top 15 journals according to the number of papers. (e.g., \"destin\" for \"destination\" and \"destinations\").