## 4 Current Issues of Last Mile Logistics
### Environmentally Sustainable LML  
As the environmental impact of operational activities in industries has become an urgent global issue, environmental sustainability is now essential in the management and operations of the entire supply chain [71]. In particular, LML involves various externalities such as gas emissions, air pollution, noise, and congestion [11]. The significant role of LML in the environmental sustainability of a supply chain has urged the transformation of the conventional LML into a more environmentally sustainable system. Accordingly, various studies have adopted environmental factors in modeling LML problems along with traditional time and cost factors through proposed LML approaches (Table 5). \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**LML-Related Issue** & **Research Finding Integrating Proximity Stations/Points** & **Reference** \\ \hline Integrating proximity stations/points into the existing LML & Improvement of delivery time, increase in average travel speed, and reduction in greenhouse gas emissions by using proximity stations/points & [62] \\ \hline Identifying potential locations for proximity stations/points & Network min-cost flow problem with pick-own-parcel stations to maximize resources using a collaborative approach & [64] \\  & Evaluation of using parcel lockers in the Polish InPost Company system & [26] \\ \hline Routing problem for last mile delivery hubs & Location-routing model to determine the placement of last mile delivery hubs & [65,66] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: LML-related issues and research findings: Proximity stations/points and hubs. The importance of environmental sustainability in LML led to multi-criteria decisions to simultaneously consider both economic and environmental factors that have trade-offs in LML. Ref. [72] provided a conceptual framework to evaluate last mile options from economic, social, and environmental aspects that involve design criteria and performance attributes for industrial, institutional, and consumer group stakeholders. Ref. [73] proposed a bi-criteria auction process to assign last mile delivery orders to trucks that maximizes both the economic and environmental sustainability of LML. Ref. [74] evaluated three LML strategies (i.e., centralized distribution network, decentralized distribution network through home-delivery, and decentralized distribution network based on crowd logistics) through a system dynamics simulation and its multi-criteria decision analysis using the Preference Ranking Organization METHod for Enrichment of Evaluations (PROMETHEE) on economic, technological, environmental, and societal criteria. Their case study of local food LML showed that a distributed network based on crowd logistics is the most sustainable LML strategy. The sustainability role of LML in e-commerce was also widely discussed to suggest new sustainable LML strategies in the literature. Ref. [43] addressed that last mile deliveries through e-commerce resulted in lower carbon footprints than conventional brick-and-mortar stores. Ref. [75] addressed that local collection and delivery points for failed home deliveries of e-commerce can effectively reduce greenhouse gas emissions against traditional shipping methods. Ref. [76] developed a stochastic LML model to calculate probabilistic estimates of traveling distances, and they compared carbon emissions generated by conventional customer pickup and last mile delivery through e-commerce. Ref. [77]