*5.2. Limitations*

The current study might not have exhausted all relevant variations of container technologies. As further examples, one could consider double door containers, where doors are available on both short sides of the container. This means that the container can be loaded while using one of the doors in the pickup-region and unloaded using either of the doors in the delivery-region, implying that there is a choice between last-in-first-out loading and first-in-first-out loading. Yet, other variants can be derived using open top containers, where goods are loaded and unloaded from the top side of the container, given that suitable equipment for loading and unloading is available at each customer location.

Different loading decisions may require the consideration of load stability, depending on the weight of the goods transported. Consider as an example Figure 2: if each pallet has a significant weight, a load as indicated might lead to dangerous situations on the road, as the content of the container is much heavier on one side. The issue of stability might be more critical when using open side containers, but the increased flexibility might nevertheless be economically attractive, at the expense of dealing with an optimization problem that has additional constraints to ensure stability at each leg of the routes.

**Figure 2.** Illustration of stacking with *R* = 3 and *L* = 11 that may violate stability restrictions if heavy goods is transported.

Another issue not tackled in current research is when some customers have more than one pallet to be transported. The customer might require that only one visit is made, but might also require that different pallets are delivered to different locations.

**Author Contributions:** Conceptualization, L.M.H. and G.T.; methodology, G.T. and Á.F.; software, G.T. and Á.F.; validation, G.T.; formal analysis, G.T. and L.M.H.; investigation, L.M.H., G.T. and Á.F.; resources, G.T. and Á.F.; data curation, G.T.; writing—original draft preparation, L.M.H.; writing—review and editing, G.T. and Á.F.; visualization, L.M.H.; supervision, L.M.H.; project administration, L.M.H. and G.T.; funding acquisition, L.M.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** The first author was supported by the AXIOM project, partially funded by the Research Council of Norway. The second author was supported by the Government of Spain, gran<sup>t</sup> MTM2015-65803-R, and the local Government of Madrid, gran<sup>t</sup> S2013/ICE-2845 (CASI-CAM-CM).

**Acknowledgments:** The authors wish to thank the three anonymous reviewers that contributed with useful inputs to help improve the manuscript.

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