Environmental Engineering Reference
In-Depth Information
• The scenario with RZ = 9 and TW = 2 shows better results than the scenario
with RZ = 5 and TW = 6. This indicates that the influence of the length of the
time window increases as the size of restricted zone grows.
Based on the results provided by the 30 scenarios analyzed in the case study, we
can conclude that the VRPATW algorithm constitutes a sound technique to assess
the introduction of access time window policies from the point of view of the extra
costs imposed on carriers. We have shown the influence of the three parameters
considered (number of customers to visit, size of the restricted zone and length of
the time window) and the fact that a small increase in one of them does not
significantly affect the results, but bigger increases in at least two of them causes
relevant extra costs.
These conclusions cannot be considered obvious when these extra direct costs
are neglected by local authorities when implementing access time window poli-
cies, while considering externalities like pollution, congestion or visual intrusion.
The contribution of Operations Research techniques provides a powerful tool to
evaluate these extra costs and incorporate them to the analysis.
4 Conclusions
Access time windows constitute a commonplace policy for local authorities to
restrict the movements of delivery vehicles in central urban areas. They are easy
and cheap to implement and to enforce, with the objective of avoiding the over-
lapping of peak hours for delivery vehicles and private cars. Thus, the social
benefits of this type of policy are clear, reducing congestion, parking problems and
noise peaks. However, there exist also a series of additional costs, which are
imposed on carriers and which are often not brought into the picture. These costs
are related to the need to use a larger number of trucks in the fleet and to make
those trucks drive longer distances. The consequence is that a policy destined to
reduce pollution and congestion might in the end be causing higher levels of
pollution and congestion.
Therefore, a procedure to estimate those costs and distances is needed in order
to incorporate the corresponding negative effects to the overall cost-benefit anal-
ysis. Previous estimation efforts have used the classical VRPTW approach, which
enables the analyst to employ standard solution techniques and software packages.
However, the vehicle routing problem with access time windows (VRPATW) is
not so easily transformed into a VRPTW. In the latter, time windows are assigned
to specific nodes (customers), while in the former it is a whole area of the network
that is subject to those time restrictions, with the subsequent effect on routing and
waiting practices. This is why our VRPATW development proves useful in the
analysis of access time window policies, allowing city planners and transportation
companies to determine the extent of the extra costs and distances imposed on
carriers by those access restrictions.
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