Direct Effects of City Logistics Measures and Urban Freight Demand Models - Sustainable Urban Logistics: Concepts, Methods and Information Systems - page 214

This model gives us the probability that the end consumer, arriving in a given
zone, purchases something of a certain dimension (dim). Therefore, the quantity of
freight type s sold by retail outlets k in zone d, Q .d [sk], can be calculated as:
Q : d s ½¼ X
i

Q : d s ½¼ X
i

X

D i od skm p i ½ ½ dim = mks dim ð 2 Þ

o ; m ; dim

where

• Q : d s ½ is the goods quantity bought/sold in retail outlet k in zone d given by the
demand of end consumers belonging to category i living/working in a zone
within the study area;
• dim is the dimension of purchases, expressed in kg;
• p i [dim/mks] is the probability that a trip concludes with a purchase of dimension
dim conditional upon undertaking a trip to retail outlet k for a purchase of goods
type s using transport mode m.

Trip generation is mainly affected by socio-economic characteristics and land-
use patterns (or the physical characteristics of the area; Cubukcu 2001 ; Cao et al.
2010 ). The current models propose to estimate the number of trips undertaken by
the individual category i departing from a zone o for shopping by two main
categories of models: behavioral (or more properly, random utility models; Russo
and Comi 2012 ) and statistic-descriptive models (Gonzalez-Feliu et al. 2010 ;
Comi and Conte 2011 ).
As stated above, few models have been proposed to investigate the choice of
shop type and hence some models of this type are proposed according to different
freight types and socio-economic characteristics of end consumers.
There are several methods to model trip distribution (Ortuzar and Willumsen
1990 ; Cascetta 2009 ) with multinomial logit structures. Amongst others, Jang
( 2005 ) used joint disaggregated models to describe the generation and distribution
of shopping trips. Veenstra et al. ( 2010 ) proposed an aggregated gravity model to
simulate trip distribution for the shopping purpose. Comi and Conte ( 2011 )
developed gravity models according two different freight types, i.e. durable and
non-durable goods. Finally, Gonzalez-Feliu et al. ( 2012 ), assuming that the choice
of purchasing location occurs simultaneously with modal choice, obtained O-D
shopping trips by first using regressive models to simulate the attracted trips for
purchasing; secondly, the origin of trips is simulated by a gravity model. They
refer only to the car mode.
Mode choice is a typical example of a travel choice that can be modified for
different journeys in which performance or level-of-service attributes have con-
siderable influence. Some examples on the investigation of shoppers' attitudes
toward the various transport modes for shopping purposes are in Williams ( 1978 )
and Cervero ( 1996 ). Finally, some researchers also propose to model destination
and mode choices jointly (Richards and Ben-Akiva 1974 ; Vrtic et al. 2007 ), or
mode and departure choices (Bhat 1998 ).