Planning and Optimization Methods for Advanced Urban Logistics Systems at Tactical Level - Sustainable Urban Logistics: Concepts, Methods and Information Systems - page 150

same stage. This problem can present several variants (Gonzalez-Feliu 2008 ;
Mancini 2011 , 2013a ). Note that in more realistic situations, the basic version
needs to be extended to a multi-depot, multi-carrier heterogeneous fleet case. In
other words, more than one starting depot (corresponding to a CDC) is defined
(Nguyen et al. 2012 ), and vehicles can be of different characteristics for the same
stage (Gonzalez-Feliu and Salanova 2012 ). Moreover, more than one carrier can
share platforms or even vehicles (Gonzalez-Feliu et al. 2010 ; Gonzalez-Feliu and
Salanova 2012 ; Thompson and Hassall 2012 ). From those extensions of the basic
version, other variants can be defined to include different constraints related to
urban freight transport (Deflorio et al. 2012 ). More in details, variants may be
grouped following a classification where we consider three main aspects: network
and service features, and route limitations.
Route limitations are applied to one or more routes, on one or more stages. Two
types of limitations can be considered, and are distance and time constraints. A 2-
stage distance constrained VRP is a variant of the basic problem of the same
family where one or more k-stages present maximum distance limits. These limits
are expressed in terms of maximum distance that vehicles can travel, and they will
be related to the vehicle's characteristics. This distance can be explained in terms
or travel distance (in km), or in terms of travel time. In this second group of
constraints, different factors like, times related to loading and unloading opera-
tions, and slack pauses can be considered, and represent the maximum time a
vehicle can be on service, for maintenance, crew working hours and other reasons.
In network features variants, the main important problem should be the 2-stage
multi-depot VRP. This problem, analogously to classical VRP, presents more than
one depot, so the starting point of each 1st-stage route can be different to the
others. In these problems, two main policies are considered. The first is that the
freight type is the same for all customers and all depots have an enough quantity of
freight to serve all customers. Analogously to MDVRP, in ME-MDVRP, freight
requested can be assigned to one of the available depots. Additional constraints
can be added to the depot availability, as for instance the depot capacity, time
period for service (defined by the opening and closing hours which correspond to
the limits beyond which it is not possible to arrive to the depot), but in all cases it
is supposed that all customers can be assigned to all depots.
Service features variants refer to some aspects which the distribution service
company offers in the transportation service. Two main families of variants are
presented, analogously to classical VRP. The first of, and maybe the most
important, due to time limitations, is the ME-VRP with Time Constraints. Several
types of time constraints, which represent different temporal aspects of multi-
echelon transport organization, can be considered. We will describe those which
can be observed in most real applications where time constitutes one of the main
factors describing the proposed service features.
The most common time limitation, analogously to classical VRP and distri-
bution problems, is the Time Window (TW) in which the vehicle can visit a
facility. The problem is called NE-CVRP with time windows (NE-VRPTW), and
the

TW are associated to nodes (usually, time limitations are not directly