Direct Effects of City Logistics Measures and Urban Freight Demand Models - Sustainable Urban Logistics: Concepts, Methods and Information Systems - page 215

4.3 Restocking Trip Models

Referring to the general modeling framework proposed by Nuzzolo and Comi
( 2013c ), the following sections describe a model sub-system that allow us to
estimate quantity, delivery and freight vehicle O-D flows with characteristics of
restocking tours. Although they can refer to different freight types s, for simplicity
of notation, the class index s will be understood.
Let Q od [k] be the average quantity of restocking flows moved from zone o (e.g.
warehouse location zone) to the retail outlets of type k of zone d; it can be
estimated as follows:

Q od ½¼ Q : d ½ po = dk ½ ð 3 Þ

where

• Q .d [k] is the average freight quantity attracted by (i.e. to be delivered in) zone
d and retail outlet k, obtained by the shopping trip models (see Eq. 2 );
• p[o/dk] is the probability that freight attracted by zone d and retail outlet
k comes from zone o; it represents the acquisition share obtained by an
acquisition model.

Having obtained the O-D flows in terms of quantities, O-D freight vehicle flows
are required for the forecasting of performances and impacts (Fig. 1 ). Different
types of methods and models have been proposed, starting directly from O-D
quantity flows (Raothanachonkun et al. 2007 ; Wang and Holguin-Veras 2008 )or
introducing further steps in order to characterize the quantity flows in terms of
deliveries (Routhier and Toilier 2007 ; Nuzzolo and Comi 2013a ). This stage is
required if we want to investigate the effects related to implementation measures
impacting on the type of service (own account or using third party vehicle) and
shipment size.
After estimating O-D flows in terms of deliveries, the next step is to convert
them into tours and hence into O-D freight vehicles (Fig. 2 ). The vehicle level
aims to do precisely that. The translation from deliveries to vehicles is not direct,
particularly in urban areas where freight vehicles undertake complex routing
patterns involving trip chains (tours). Indeed, each restocker jointly chooses the
number and location of deliveries for each tour and hence defines his/her tours,
trying to reduce the related costs (e.g. using a routing algorithm). The freight
vehicle O-D matrices are then obtained from the delivery O-D matrices using a
two-step procedure as proposed by Nuzzolo and Comi ( 2013a ): definition of
delivery tours from delivery O-D matrices, definition of freight vehicle O-D
matrices from delivery tours. For more details on the above model calibration and
validation, refer to Nuzzolo and Comi ( 2013a and c ).