Planning and Optimization Methods for Advanced Urban Logistics Systems at Tactical Level - Sustainable Urban Logistics: Concepts, Methods and Information Systems - page 146

(a) (b)

(c) (d)

Fig. 2 A comparison between a single-tier (a) and three two-tier (b-d) distribution systems

The different operations in two-tier CL systems (command preparation, con-
solidation, sorting, etc.) are performed at facilities organized into a hierarchical
structure, as illustrated in Fig. 2 : major logistics terminals and depots are located
in the urban periphery, in urban consolidation platforms, called here city distri-
bution centers (Crainic et al. 2012 ) where it is loaded into urban trucks, which are
of medium dimensions (ideally 9-12 t); then freight is transported city freighters
(3.5) at crossdocking satellites, strategically located close to or within the city
centre, from where final customers are delivered. Satellites are generally intended
to be simple transhipment facilities like vehicle reception points (Boudouin et al.
2013 ) and operate according to a vehicle-synchronization and cross-dock tran-
shipment model (Drexl 2012 ), i.e., urban vehicles and city freighters meet at
satellites at appointed times, with only short waiting times being permitted.
Indeed, no intermediate storage is allowed at satellites if not for a very small time
(in general less than half an hour).

2.3 Challenges and Opportunities

Although urban consolidation centers have been studied by many researchers, the
derived systems present several challenges and opportunities for city logistics
research, more precisely related to operations research and management sciences.