Behavioral Modeling of Urban Freight Transport - Sustainable Urban Logistics: Concepts, Methods and Information Systems - page 235

the agent would be willing to receive for not having potentially gained from the
increase in the level of the beneficial attribute under consideration.
Notwithstanding the interesting analysis just discussed one has to scrutinize the
policy implications derived by using either M1 or M2. An informative comparison
between the WTP estimates (and their respective confidence intervals) of the two
models is reported in Table 7 .
We underline that all the results reported use Beta EF3-4 as a base since this
represents the part-worth utility variation from the SQ (600€) to the next step up
(800€). Moreover, for M2, having effects coded the variables, one has to be careful
in interpreting results especially when it comes to WTP measures. In fact, one
should recall that the WTP to move from the basic level of an attribute to a
different one represents the difference in the corresponding valuations (Collins
et al. 2012 ). In our case, for LUB3 we have 113€ representing the amount of
money interviewees are willing to pay to obtain 800 additional LUB.
The results reported in Table 7 show the strong policy impacts that adopting
either a linear or non-linear assumption might have. In fact, comparing the results
of M1 and M2, one observes substantial differences for both LUB and PLUBF. In
particular, Fig. 3 shows that M1 tends to overestimate WTPs associated with the
greatest attribute levels and, in these cases, the distributions are much flatter than
those related to M2 (i.e. LUB3 and PLUBF3). On the contrary, M1 underestimates
WTP linked to intermediate level and the distribution is characterized by a little
dispersion around the mean value (i.e. PLUBF2).
From a purely statistical point one should suggest policy maker to have more
faith in M2 results giving its higher explanatory power given its capability to fit the
data.
5 Concluding Remarks
This paper reports the results from an empirical research on UGM policy inter-
vention in the Roman freight LTZ. The research specifically focuses on retailers'
preference analysis for hypothetical policy scenarios. The paper innovates in terms
of questionnaire development and in terms of ex-ante policy-mix evaluation. The
results obtained are relevant both from a theoretical point of view as well as from a
more practical and policy-oriented perspective. It is noticeable that notwith-
standing the often called for agent-level analysis, the literature on UGM policies
has rarely investigated this issue at this specific level. Therefore, the paper rep-
resents a first attempt at bridging the gap between theory, applied research and data
needs.
In more detail, from a methodological stance the results reported show that not
only it is important and interesting to adopt an agent-based point of view but also
to consider potentially non-linear effects of the policy instruments adopted. Data
reveals, in fact, that both with respect to all attributes considered the policy
potentially implemented might have a different effect depending on the attribute