Safety Vision of Agricultural Tractors: An Engineering Perspective Based on Recent Studies (2009–2019)

The high rate of injuries occurring in agricultural activities is of major concern in most countries, despite the ever-increasing efforts made at normative levels. In particular, the use of agricultural tractors is recognized as the most hazardous activity for farmers due to the large number of fatalities occurring every year. The aim of the present study was to investigate the recent developments in research activities dealing with tractor safety. For this purpose, a systematic literature review was carried out, taking into account engineering journal papers appearing in Scopus in the 2009–2019 period and focusing on tractor safety. As a result, 79 documents were selected and analyzed based on both their type (e.g., conceptual or empirical studies) and specific targets. They were then classified and discussed in accordance with a reference framework representing the main issues of agricultural tractor safety: mechanical hazards, protective devices, command and control, other hazards, ergonomics, information, conformity, and user behavior. The results of this analysis brought to light the need for a more human-centered approach when dealing with tractor safety. In addition, the lack of a reliable framework of technical standards was also stressed. Overall, despite the limitations due to the selection criteria, this study represents the first systematic literature review depicting the status of tractor safety in the engineering field, providing a basis for further research on the emerging themes outlined.


Introduction
In recent years, awareness of occupational health and safety (OHS) issues has increased worldwide thanks to both the efforts made by national and international legislators, who have developed stricter and more thorough normative frameworks, and the spread of new approaches to safety problems in workplaces relying on "safety culture" and "safety management" concepts [1][2][3][4]. However, the number of occupational accidents and fatalities is not decreasing sufficiently. For example, as reported by Ivascu and Cioca [5], in European Union (EU) countries, the number of non-fatal incidents (i.e., those causing at least four calendar days of absence from work) was 3,127,546 in 2013, while in 2017 it reached 3,315,101 cases. In these statistics, agricultural activities represent one of the most hazardous sectors. As confirmed by a recent report from the European Agency for Safety and Health at Work (EU-OSHA), fatalities in agriculture are three times higher than the average [6]. In non-European countries, this situation is very similar, including in the United States [7], Canada [8], Australia [9], and Korea [10].
The main causes of this phenomenon can be summarized as follows: the large number of small-sized and family-run companies; the great variety of outdoor activities carried out by the This procedure is consistent with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [43], which are a widely recognized tool for review studies in the safety field [45]. Following the research goal defined in the previous section, the studies registered in the Scopus database from 2009 to 2019 (up to September 2019) were considered, first selecting those papers with keywords including the following search strings: "tractor" and "safety" (Research A); "machine*" and "safety" and "agricultur*" (Research B); "machine*" and "safety" and "farm*" (Research C). The results of this first step consisted of 275 documents for Research A, 230 for Research B, and 53 for Research C (558 documents in total). Next, journal articles (i.e., excluding conference papers and book chapters) published in English were selected and among those, those classified in the "Engineering" field were filtered, returning 109 (Research A), 53 (Research B), and 15 (Research C) documents (177 in total). The details of this search are provided in the Supplementary Materials. The main reasons for the above criteria, according to which the first screening was carried out, are as follows. The Scopus database is one of the most diffused and well-known databases of peer-reviewed academic and scientific documents, of which the performance, especially in the natural science and engineering fields, are considered comparable or superior to other databases of peer-reviewed articles [46]. The choice to limit the analysis to the engineering field aimed to provide a specific point of view of a problem that at a general level entangles different subject areas (and journals), such as agronomy, medicine, engineering, etc. The definition of a specific subject area is a key factor for performing a reliable systematic literature review [47].
Further filtering was carried out by reading the abstracts of all these documents and selecting those addressing agricultural (four-wheeled and tracked) tractor safety issues. Thus, studies focusing on lawn tractors, off-road utility vehicles, gardening machinery, two-wheeled tractors, and similar types of machinery/vehicles were excluded, as well as those articles discussing OHS issues among farmers in general. Conversely, papers discussing safety training of the users of agricultural tractors were taken into account in the analysis, as this training is mandatory in most countries and, in some cases (e.g., in Italy), a specific competency qualification of professional users is required. The output of this analysis led to a final list of 79 documents; an overview of the selection process is shown in Figure 1.

2019, 6, x FOR PEER REVIEW 3 of 20
This procedure is consistent with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [43], which are a widely recognized tool for review studies in the safety field [45]. Following the research goal defined in the previous section, the studies registered in the Scopus database from 2009 to 2019 (up to September 2019) were considered, first selecting those papers with keywords including the following search strings: "tractor" and "safety" (Research A); "machine*" and "safety" and "agricultur*" (Research B); "machine*" and "safety" and "farm*" (Research C). The results of this first step consisted of 275 documents for Research A, 230 for Research B, and 53 for Research C (558 documents in total). Next, journal articles (i.e., excluding conference papers and book chapters) published in English were selected and among those, those classified in the "Engineering" field were filtered, returning 109 (Research A), 53 (Research B), and 15 (Research C) documents (177 in total). The details of this search are provided in the Supplementary Material. The main reasons for the above criteria, according to which the first screening was carried out, are as follows. The Scopus database is one of the most diffused and well-known databases of peer-reviewed academic and scientific documents, of which the performance, especially in the natural science and engineering fields, are considered comparable or superior to other databases of peer-reviewed articles [46]. The choice to limit the analysis to the engineering field aimed to provide a specific point of view of a problem that at a general level entangles different subject areas (and journals), such as agronomy, medicine, engineering, etc. The definition of a specific subject area is a key factor for performing a reliable systematic literature review [47].
Further filtering was carried out by reading the abstracts of all these documents and selecting those addressing agricultural (four-wheeled and tracked) tractor safety issues. Thus, studies focusing on lawn tractors, off-road utility vehicles, gardening machinery, two-wheeled tractors, and similar types of machinery/vehicles were excluded, as well as those articles discussing OHS issues among farmers in general. Conversely, papers discussing safety training of the users of agricultural tractors were taken into account in the analysis, as this training is mandatory in most countries and, in some cases (e.g., in Italy), a specific competency qualification of professional users is required. The output of this analysis led to a final list of 79 documents; an overview of the selection process is shown in Figure 1. Data analysis and synthesis were then carried out by going through the documents' full text. In doing this, the approaches proposed by Carnevalli and Cauchick Miguel [48] and Verbano and Venturini [49] were used, tailored to the specific context of this study. Accordingly, the papers obtained from the selection process were classified from different points of view: Data analysis and synthesis were then carried out by going through the documents' full text. In doing this, the approaches proposed by Carnevalli and Cauchick Miguel [48] and Verbano and Venturini [49] were used, tailored to the specific context of this study. Accordingly, the papers obtained from the selection process were classified from different points of view:

1.
Type of publication: i.e., the engineering journals that published the selected documents, the affiliation country of the first author when the paper was published, the year of publication, etc.

2.
Research category: distinguishing the selected documents into empirical and conceptual works. In detail, adapting the criteria provided by the above-mentioned studies [48,49] to the context of agricultural tractor safety, we considered: • "empirical study": a research work focused on the development of novel technical solutions, the practical test of the performances of the tractor and its components, as well as articles undertaking surveys and case studies; • "conceptual study": a research work aimed at providing theoretical concepts, review works, simulations, or theoretical modeling.
Based on this, a more detailed classification was performed, dividing empirical studies into the following subcategories: • E-S: empirical study based on surveys and interviews; • E-T: empirical study based on practical testing of real equipment and user behavior; • E-N: empirical study providing a novel technical solution, e.g., a prototype.
As far as conceptual works are concerned, the following subcategories were defined: • C-D: studies based on data collection and analysis (e.g., accident statistics); • C-M: studies providing software simulation and modeling; • C-R: works focusing on a literature review.
These sub-categories refer to the target of each study and the means used to achieve it. For instance, if an article proposed an assessment model for which validation is carried out through a practical case study, the paper was classified as C-M (and not as E-T), since it aimed to provide a conceptual output.

3.
Research streams: i.e., the classification of documents in accordance with a reference framework representing the main issues of agricultural tractor safety. This analysis was aimed at investigating the research trends related to tractor safety in the engineering field, underlining the aspects dealt with by the selected documents [50,51]. For this purpose, as a classification benchmark, the following research streams were used: S.1. Mechanical hazards (stability, mobility, entanglement, etc.); S.2.
Conformity (i.e., compliance with technical standards and mandatory requirements both from the constructive and users' standpoints); S.8.
User behavior (safety attitudes and behavior of users, human error, etc.).
These research streams were selected following the approach proposed by the Machinery Directive [52], which in its Annex I provides a clear framework of safety requirements related to machinery and its mobility. This choice was also motivated by the nature of the selected papers, which dealt with machinery safety in an engineering context. Hence, while one might reasonably argue that subjects such as safety signs or vibrations should be included in the ergonomics area, the use of the Machinery Directive approach as reference framework allowed us to use a widely recognized scheme Safety 2020, 6, 1 5 of 20 from an engineering point of view. This included the research stream S.8. (user behavior) reflecting the need to take into account the "reasonably foreseeable misuse" or readily predictable human behavior of machinery users. This aspect is strictly related to the design and risk assessment of machinery, which require a human-centered approach [53] to consider human behaviors performed in work activities related to the use of equipment, taking into account foreseeable errors during its operation [54][55][56]. Similarly, training activities of tractor users are considered a relevant topic in machinery safety [57] and were included in the stream S.6. Moreover, this classification was in line with safety issues related to agricultural tractors provided by the International Labour Office (ILO) Code of practice concerning safety and health in agriculture [58], which is recognized by many studies as a worldwide reference document for OHS in agricultural activities [59,60].
To make this analysis more reliable and replicable, it was carried out independently by both the authors, in line with the research cues suggested by Vinayak and Kodali [61].

Type of Publication
The selected documents were published by 25 different engineering journals covering a wide range of engineering topics ( Table 1).
The distribution of publication year in the selected period is reported in Figure 2, where the highest numbers of publications belonged to 2018 and 2016. In 2018, the number of journals where the documents were published was greater, while in 2016 the larger number of publications belonged to a specific special issue [62].
Concerning the affiliation of the first author, as shown in Table 2, the majority of papers were published by researchers from the USA and European countries (mainly Italy and Spain). Concerning the affiliation of the first author, as shown in Table 2, the majority of papers were published by researchers from the USA and European countries (mainly Italy and Spain).

Research Categories
The next step of the analysis consisted of identifying the specific objective of each documentby going through the text, the goal of the studies was identified. This also allowed us to categorize them following the criteria explained in the previous section. Figure 3 reports the number of empirical and conceptual studies is reported in percentages. Within the subcategories, most empirical studies discussed the results of surveys among tractor users and experts, while the majority of conceptual works were focused on simulations and modeling ( Figure 4). Among the latter only one review paper was found, concerning the training programs for the safe use of tractors and agricultural machinery among youth [63]. This supports the research

Research Categories
The next step of the analysis consisted of identifying the specific objective of each document-by going through the text, the goal of the studies was identified. This also allowed us to categorize them following the criteria explained in the previous section. Figure 3 reports the number of empirical and conceptual studies is reported in percentages. Concerning the affiliation of the first author, as shown in Table 2, the majority of papers were published by researchers from the USA and European countries (mainly Italy and Spain).

Research Categories
The next step of the analysis consisted of identifying the specific objective of each documentby going through the text, the goal of the studies was identified. This also allowed us to categorize them following the criteria explained in the previous section. Figure 3 reports the number of empirical and conceptual studies is reported in percentages. Within the subcategories, most empirical studies discussed the results of surveys among tractor users and experts, while the majority of conceptual works were focused on simulations and modeling ( Figure 4). Among the latter only one review paper was found, concerning the training programs for the safe use of tractors and agricultural machinery among youth [63]. This supports the research Within the subcategories, most empirical studies discussed the results of surveys among tractor users and experts, while the majority of conceptual works were focused on simulations and modeling ( Figure 4). Among the latter only one review paper was found, concerning the training programs for the safe use of tractors and agricultural machinery among youth [63]. This supports the research motivations of the present study, highlighting the lack of thorough literature reviews on tractor safety. motivations of the present study, highlighting the lack of thorough literature reviews on tractor safety.

Research Streams
Finally, the research issues addressed by the selected articles were investigated. The analysis consisted of defining the research target of each document, i.e., which topics were investigated by the studies in order to enhance tractor safety. The results of this analysis are summarized in Figure 5, which shows that most studies dealt with conformity and user behavior issues, whereas documents addressing hazards due to mechanical and other sources were less numerous. The full list of the documents with the assigned reference number, subcategory codes and the related research stream is reported in Appendix A, while the following subsections provide a more detailed description of them.

Research Streams
Finally, the research issues addressed by the selected articles were investigated. The analysis consisted of defining the research target of each document, i.e., which topics were investigated by the studies in order to enhance tractor safety. The results of this analysis are summarized in Figure 5, which shows that most studies dealt with conformity and user behavior issues, whereas documents addressing hazards due to mechanical and other sources were less numerous. The full list of the documents with the assigned reference number, subcategory codes and the related research stream is reported in Appendix A, while the following subsections provide a more detailed description of them.

Research Streams
Finally, the research issues addressed by the selected articles were investigated. The analysis consisted of defining the research target of each document, i.e., which topics were investigated by the studies in order to enhance tractor safety. The results of this analysis are summarized in Figure 5, which shows that most studies dealt with conformity and user behavior issues, whereas documents addressing hazards due to mechanical and other sources were less numerous. The full list of the documents with the assigned reference number, subcategory codes and the related research stream is reported in Appendix A, while the following subsections provide a more detailed description of them.  [66]. A prototype of a triangular rubber track unit to replace tires was proposed by Franceschetti et al. [67] to reduce the probability of loss of stability, whereas other studies investigated the reliability problems related to the three-point hitching system to use the maximum hydraulic lifting capacity safely [68], as well as the improvement of the linkage and control valves [69].

Protective Systems
The research on protective systems of tractors is largely characterized by studies on ROPS. These studies aimed to augment the structural protection effectiveness by analyzing the maximum mass, which is used as reference mass in the Code 4 and SAE J2194 Standard tests [70]; the behavior of size-extended models [71]; and the energy dissipation capability augmented by the insertion of an energy absorption disc [72]. Other studies investigated the performances of novel types of ROPS aimed at preventing the unsafe behavior of users, such as the AutoROPS [73] and the compact rollover protective structure (CROPS) [74,75]. Interestingly, it should be noted that in the latter papers, the acronym "CROPS" referred to a modified ROPS for narrow-track tractors developed by Italian researchers from the Italian Workers Compensation Authority (INAIL). The same acronym was used differently in other works to indicate the "cost-effective roll-over protective structure" program by the National Institute for Occupational Safety and Health (NIOSH), which aimed to promote the retrofitting of ROPS among USA farmers with tractors without ROPS, as reported by Ayers et al. [76]. In this study, a computer-based ROPS design program was developed and tested via the design of three different types of structures. Additionally, studies dealing with the safety characteristics of the actuation force of different foldable two-post ROPS [77] and the design of a falling-object protective structure (FOPS) integrating an existing two-post rear ROPS [78] were included in the stream S.2.

Command and Control
Research on the improvement of command and control systems was essentially focused on studies related to the implementation of automated solutions for the steering [79] and hitching [80] systems. Similarly, Ruggeri et al. [81] developed a smart dashboard and machine controller to monitor the functional safety performance level of tractors. Moreover, the development of devices for monitoring the engine and other utilities of the tractor, as well as the reliability of the ball-joint-placed steering actuator and the wheel spindle [82] were proposed. Additionally, Rovira-Mas [83] developed a sensor architecture for monitoring the main tractor performance. In this research area, we also included the studies on the development of navigation systems to be used in autonomous robot tractors [84,85], as well as the study of a warning system to be used during the rearward use of the tractor [86].

Other Hazards
Regarding other types of hazard, five studies dealt with problems related to the transmission of whole-body vibrations to the operator. Catania et al. measured the exposure for the operator driving both a wheeled and a tracked tractor while performing different working activities [87]; other studies focused on the definition of an exposure index in relation to the different types of tractor [88] or to different terrain surfaces [89][90][91]. In addition, Sviridova and Sakai [92] investigated the effects of noise on the operator's cardiovascular system considering different engine speeds.

Ergonomics
Ergonomics issues were investigated by focusing different aspects. Rakhra and Mann analyzed the operator's reaction time and response errors during monitoring of rear-mounted equipment [93]. Ehlers and Field investigated the visibility features of tractors to enhance rearward visibility [94,95], while Gilad and Byran [96] focused their attention on modeling the tractor operators' field of view.
Other studies dealt with the analysis of the actuation forces needed to operate foldable rollover protective structures (FROPSs) [97,98], whereas Pessina et al. [99] compared the handling load of different narrow-track tractors' FROPSs with the related grasping area. Furthermore, two studies describing field tests aimed at setting a warning device based on the operative conditions of the tractor [100,101] were included in this research stream.

Information
Several studies investigated the information provided to the tractor users by means of safety signs and pictograms put on the machinery [102][103][104], as well as in user manuals [105]. Other studies focused their attention on training courses for tractor operators [63,106,107]. Bertacchini et al. [108] paid attention to the development of driver assistance devices and warning systems to alert users in case of hazardous situations such as the loss of stability. In this group of papers, we also included the paper by Liu and Koc [109] describing the development of a detection and emergency notification system.

Conformity
Among the selected documents, the majority focused on the conformity aspects of tractor safety and three main research substreams were found. First, the analysis of the status of compliance with safety regulations among farmers (mainly related to the installation of ROPS and seat-belts) was widely carried out [20,[110][111][112], also considering the incidence of the occurrence of rollover [113]. The second aspect concerned the conformity to technical standards, which mainly focused on ROPS testing codes such as SAE J2194 [114][115][116], OECD Code 3 [117], OECD Code 4, and OECD Code 6 [118], or their combined use [119][120][121]. Similarly, the conformity to technical standards concerning whole-body vibrations was analyzed by Kabir et al. [122]. Thirdly, the risk assessment of activities related to the use of tractors based on specific work tasks [123] and the development of risk scenarios based on the analysis of rollover fatalities were investigated [21]. Furthermore, in this research area we included studies which analyzed the problem of safety compliance of tractors at both the strategic [34,124] and market level [125], as well as the development of proper equipment for tractor testing [126].

User Behavior
The number of research works dealing with the behavior of the operators is also noteworthy. In particular, most studies included in this research stream investigated the user attitude towards safety practices and risk-taking behavior [18,[127][128][129], including their perception of safety issues [37]. Jenkins et al. [130] surveyed motivations and barriers to the safe use of tractors among farmers. Accordingly, surveys were carried out also during specific demonstrations related to ROPS retrofitting [131,132]. Di Nocera et al. [133] analyzed the tractor's operator attentional control, while Lleras et al. [134] investigated the behavior of tractor operators in rollover-prone situations. Furthermore, tasks and activities carried out by the operators when using the tractor [135] and when they were involved in overturn-related fatal and nonfatal injuries [136] were investigated.

Discussion of Research Trends
Merging the results obtained from the analysis and synthesis of the selected documents some implications can be outlined.
First, the attention paid to the behavior of users was largely investigated not only by specific studies (research stream S.8), but also across different areas, such as the development of ROPS solutions. For example, some studies investigated the behavior of compact ROPS [74,75] and auto-ROPS [73], which are both technical solutions aimed at bypassing the (foreseeable) dangerous behavior of the operator when using a tractor equipped with a foldable structure (research stream S.2). In line with this aspect, other studies addressed the proper comprehension of safety signs and pictorials [102][103][104][105], highlighting the importance given to human factors (research stream S.6). In addition, documents reporting efforts made to augment the safety culture/attitudes of farmers, promoting the use of ROPS and seatbelts, were consistent with such an address [37,127,128,130,136]. These research trends confirm that "safety is, above all, behavioral and individual" [53], promoting a human-centered approach in the field of tractor safety. Accordingly, the works of Di Nocera et al. [133] and Rakhra and Mann [93], addressing cognitive failures of the tractor operator and its reaction time and response errors, can be considered consistent with this emerging theme in recent woks on tractor safety.
Another emerging aspect from this review was related to the geographical differences among the selected documents. While most of the documents analyzed in this study focused on ROPS retrofitting, the majority of studies carried out in the USA and Australia described activities related to national programs aimed at increasing the number of ROPS retrofitted tractors through surveys among farmers and other stakeholders. Conversely, the majority of studies by European researchers that focused on ROPS (especially in Italy and Spain) were aimed at augmenting knowledge on the technical performances of aftermarket structures by means of field and laboratory tests. These works investigated ROPS testing procedures with the goal of verifying their reliability in practical contexts, especially those related to the assessment of aftermarket structures. On the one hand, studies dealing with ROPS retrofitting could be expected, since the highest number of fatalities and serious injuries with tractors are due to loss of stability (i.e., overturns) and the number of old tractors without ROPS is still large worldwide.
Two different research trends emerged, the former fostering the farmers' safety attitude, the latter focusing on the features and conformity to technical standards of ROPSs (e.g., the respect of the clearance zone), particularly for aftermarket solutions. Consequently, the following implications can be derived: The lack of studies promoting ROPS retrofitting and user training programs in other countries (e.g., in the European Union) could reveal the need to implement/increase these activities in such places. In fact, although the Directive 2009/104/EC indicates that self-propelled work equipment should be driven only by workers who have been appropriately trained in the safe use of such equipment [137], training and competence qualifications of tractor users have not been discussed sufficiently.

2.
There is a need to improve the effectiveness and usability of technical standards for ROPS testing and homologation, so as to make ROPS design and manufacturing easier and less expensive [73,76], while avoiding at the same time the failure of homologated protective structures [70]. The latter issue was stressed by different studies, shedding light on the necessity of updating technical standards such as the OECD Codes, taking into account the features of modern tractors (e.g., the tractor ballast [120]) and how they are used in practice [65,118]. Similarly, the lack of a clear reference for dynamic tests of ROPS was outlined [117,126], while the inadequacy of the standards related to FROPSs testing (e.g., concerning the grasping area [99] and the actuation torque [97]) needs to be further investigated.
Furthermore, several studies investigated novel solutions in line with the "4.0 era" [138]. In fact, some works proposed devices for monitoring the tractor components' status to augment its safe use [79,81,82,108]. Others investigated solutions to augment the visibility of the operator [64,94,95], while two Japanese works proposed advanced solutions for navigation systems for unmanned tractors [84,85].
It is noteworthy to mention the studies addressing warning and alert systems to augment the operator's risk perception [86,101], and detection and emergency notification systems in case of overturn [109]. They outlined the need to foster the integration of a warning device to alert the operator in case of hazardous situations. In particular, monitoring the tractor's stability during operation could greatly contribute to preventing fatal accidents when using narrow-track tractors equipped with FROPS. The proposal of an AutoROPS [73] could also be promising, even though the solution designed for the deployment of the structure in the event of tractor rollover was not addressed in the paper specifically.
Accordingly, the results of the present study showed that although not many studies dealt with this emerging research field, the potential to augment tractor safety by means of automated solutions relying on Industry 4.0 means are great, and deserving of further consideration.

Limitations
The results obtained from this review were certainly affected by the limitations that characterize any review study, i.e., the criteria used for searching and screening the documents, such as the target analysis, keywords, and type of publication. In particular, the choice of limiting the analysis to engineering journals only excluded numerous relevant works published in journals classified in Scopus into different subject areas, such as medicine and agricultural and biological sciences. Although the selection of documents within a specific subject area is recommended when performing a reliable systematic literature review [47,139], this certainly diminished the general validity of the study. Consequently, the output of the study cannot be considered exhaustive and it must be evaluated in that specific context [45]. However, the proposed criteria for the data analysis and synthesis can be considered a basis for further research works addressing both the agricultural context and safety issues in different sectors.

Conclusions
This study aimed to provide an overview of tractor safety in the engineering field. With this goal in mind, 79 engineering journal articles published in Scopus in the 2009-2019 period were analyzed by means of a systematic literature review approach. The identified sample of documents was deemed to sufficiently represent the recent targets and trends on tractor safety research. Moreover, the fact that these studies were very diverse in terms of sectorial (i.e., journal source), technical (i.e., the scientific approach used), and geographical background provided a realistic overview of the cutting edge in this specific context. A set of research streams based on the main issues of agricultural tractor safety was used to analyze the contents of the selected documents.
The review shed light on the emerging themes that academics and researchers are dealing with in order to reduce the number of injuries related to the use of agricultural tractors. The proposed framework for classifying the selected studies allowed us to better highlight the different engineering perspectives of tractor safety. In particular, the attention paid to retrofitting ROPS on older tractors came forth as the most relevant safety issue worldwide. The analysis revealed that different approaches have been applied, ranging from users' involvement in surveys aimed at increasing their safety attitudes and consciousness, to the implementation of computational models and field tests for the verification of testing procedures.
The study also brought to light the different user-centered approaches aimed at reducing the unsafe behavior of operators. This could provide useful insights to researchers and practitioners on potential improvements from different points of view, especially taking into account the possibilities offered by the newest technologies.
Overall, the merit of this study can be considered twofold. On the one hand, it represents a first attempt to describe up the status of tractor safety in the engineering field by means of a systematic literature review, providing a framework for the analysis and synthesis of documents in this specific context, which could allow us to bring to light research streams and emerging themes proposed in recent literature. On the other hand, since review studies can augment knowledge on specific issues, providing evidence on trends and challenges in a systematic manner [43,44], the output of this work could be used for future studies to extend the knowledge on safety issues not only in agricultural activities, but also in other contexts where the use of machinery and work equipment represents the main cause of severe injuries. Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A
List of the selected documents organized per research streams, where research categories were organized as follows: E-S: empirical study based on surveys and interviews; E-T: empirical study based on practical testing of real equipment; E-N: empirical study providing a novel technical solution, e.g., a prototype; C-D: conceptual studies based on data collection and analysis (e.g., accident statistics); C-M: conceptual studies providing software simulation and modeling; C-R: conceptual works focusing on a literature review; and research streams were individuated by the following codes: