Literature reviews are utilised to help build understanding of extant knowledge, identify gaps, and draw out concepts to frame a field. Such framing may be used to inform research or consultation questions, and then to judge those further outcomes. Most literature reviews in social sciences are ‘narrative’ reviews, developing an overview of a field through a reasonably comprehensive assessment and critical reading of the literature. An alternative approach, originating in medicine and traditional sciences is the ‘systematic’ review. This applies a transparent, replicable, and often quantitative methodology to literature identification and review. In some highly quantitative fields, a researcher may adopt a meta-analysis, extrapolating and synthesizing results from multiple papers. The use of full or partial systematic literature reviews in operations management, transport and logistics research has increased in recent years [22–26].
The Critical Appraisal Skills Program (CASP) has developed methodologies for systematic and critical reviews of literature since 1993 [27]. With roots in UK healthcare, it is not tied to any discipline and has been deployed for qualitative as well as quantitative sources [28]. The approach, which acts as an inclusion/exclusion protocol is viewed as rigorous but flexible and has in recent years been adopted by operations and transport researchers in both systematic and semi-systematic reviews [29].
It has been noted that a weakness of literature reviews in the field of transport, was the lack of explicit methodology [30]. To mitigate this, we adopted a “semi-systematic review”. This is a review guided by a systematic review in terms of literature survey and selection [31], combined with “framing a written discourse about the literature which may be established as a part of a thesis or other research” [32].
This hybrid approach allows for an emphasis on such features as transparency about searching, and the potential for comprehensiveness. These “systematic” protocols have been evidenced in use to create replicable research search procedures, for cross-checking or follow-up research in another place or at a later time.
The narrative phase of the review allowed for insight from reading and understanding to allow a “plurality of knowing [33]”. This addressed the suggestions that a metric-driven meta-analysis is not appropriate for transport-based literature reviews, due to the “fact that many variables influence an independent variable (e.g., travel behaviour) in a complex way, resulting in complex causal relationships, and a multitude of data analysis methods and interpretations” [30].
The protocol adopted is shown graphically, as a flowchart, in Figure 1 below.
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