# A Review of Literature on Vehicle Routing Problems of Last-Mile Delivery in Urban Areas
## 4 Literature on LMD Vehicle Routing
### Sidekicks  
Delivery by sidekicks in LMD is based on drones and real-time navigation technology [110]. Sidekick delivery uses main vehicles, usually trucks, which carry sidekicks and parcels to local areas, where the sidekicks complete delivery. Drones, or Unmanned Aerial Vehicles (UAVs) [51], have evolved from military applications tracing back to the 1930s. As their costs have substantially decreased, they have become readily available to the public and logistics companies in the last 10 years [111]. Sidekicks are often aerial drones with different capabilities and limitations in their carrying capacity, charging time, and coverage area. Sidekick delivery has many other names, such as drone-assisted delivery [112; 113], multi-modal delivery [13], carrier-vehicle [12], joint-delivery systems [114], two-echelon delivery [115], and automated delivery robots [116]. Fleet synchronization and collaborative operations between main vehicles and drones are important. Among the 10 categories of two-echelon VRPs in [117], synchronization \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Ref.** & **Problem Nature** & **Important Considerations** & **Objectives(s)** & **Sample Size** & **Solution Method** \\ \hline \multirow{4}{*}{[104]} & \multirow{4}{*}{-} & & - & Use of customers who come to location of lockers as ODs for home delivery requests for specific customers) & Minimize the cost of locker supply, OD if they act as ODs & Minimize the cost of lockers & \multirow{4}{*}{-} & \multirow{4}{*}{40 to 106 customers - - 3 to 5 lockers} & \multirow{4}{*}{Exact solve with CPLEX} \\  & & & - & Limited capacity for the lockers, fleet, and ODs & & & & \\  & & - & Sub-tours are not allowed & & & & \\ \hline \multirow{4}{*}{[106]} & \multirow{4}{*}{-} & & - & Waiting time for the customers & \multirow{4}{*}{-} & \multirow{4}{*}{Minimize parcel waiting time} & \multirow{4}{*}{-} & \multirow{4}{*}{30 customers} \\  & & & - & Effect of naval services and ferries & & & & \\ \cline{4-7}  & & & - & Specified capacity for lockers & & & & \\ \hline \multirow{4}{*}{[107]} & \multirow{4}{*}{-} & & - & Parcel locker is an option for customers & \multirow{4}{*}{-} & \multirow{4}{*}{25, 50, 75 customers} & \multirow{4}{*}{Heuristic large neighborhood search} \\  & & & - & Home delivery is limited to time windows & & & & \\ \cline{4-7}  & & - & Multi-size lockers & & & & \\ \cline{4-7}  & & - & Compensates customers & & & & \\ \cline{4-7}  & & & - & Putting multiple parcels in one locker & & & & \\ \hline \multirow{4}{*}{[108]} & \multirow{4}{*}{-} & & - & Considers mobility of parcel lockers & \multirow{4}{*}{-} & \multirow{4}{*}{5, 15, 30, 50 customers} & \multirow{4}{*}{Huristic neighbor search} \\  & & & - & Using an autonomous vehicle or a human & & & \\ \cline{4-7}  & \multirow{4}{*}{-} & Capacitated & - & Limited customer travel distance & & & \\ \cline{4-7}  & & - & Limited capacities for vehicles & & & & \\ \cline{4-7}  & & - & Minimum overlap between vehicle waiting time and customer & & & & \\ \cline{4-7}  & & & - & Parking space limitation & & & \\ \hline \multirow{4}{*}{[109]} & \multirow{4}{*}{-} & & - & Total travel distances of fleet and customers & \multirow{4}{*}{Minimize travel distance cost from defects with small and medium capacities} & \multirow{4}{*}{-} & \multirow{4}{*}{65 customer zones} & \multirow{4}{*}{Metaheuristic (customer clustering \& TSP solving)} \\  & & & - & Limited and multiple defects with small and medium capacities & & & & \\ \cline{4-7}  & & - & Customer grouping cost to serve customers & & & & \\ \cline{4-7}  & & & locations. & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._between two echelons applies to the sidekick delivery. In urban areas, other options, such as bikes and postal couriers, can also be sidekicks. As shown in Figure 5, there are two different types of route in a sidekick delivery system.