Significado en los contextos de ocio.

is a relevant part of integrating sustainability dimensions into the decision criteria

(Kiba-Janiak, 2016).

Consequently, sustainability pillars are starting to be considered as decision cri- teria in distribution processes. While economic impacts can be measured through increases in operational costs, both social and environmental assessments tend to be subjective. As before mentioned, the externalities are perceived as economic in- dicators in order to measure the performance and impacts of transport activities. Besides, the economic perspective allows to take the negative impacts into account in the decision-making (Ranaiefar and Amelia, 2011). Economic, environmental, and social impacts are strongly interrelated (McKinnon et al., 2015). Prevention and mitigation costs for negative impacts need to be considered in financial reports. Prevention costs are due to the economic regulations associated with natural re- sources consumption or pollutant emissions. These costs are typically imposed by governments to avoid, or minimize, social and environmental consequences of trans- port operations. Regarding mitigation costs, they are related to penalties associated with the generation of more emissions than allowed (Santos et al., 2010). In addi- tion, companies design the preventive and mitigating actions implementing focus on sustainable strategies; for instance, the use of alternative fuel vehicles (Scheuer,

2005).

Given the importance of these facts, the European Union sustainable develop- ment strategy defines sustainable transport as one of its seven key challenges. In this context, the increasing social concern is compelling companies to change purely commercial objectives in order to consider sustainability. This new vision seeks to compensate the negative impacts of transport activities without neglecting economic profits. Despite the fact that the literature on transport is extensive, there is a lack of works on urban transport taking into account social and environmental issues simultaneously (Geurs et al.,2009).

1.4

VRP variants for the sustainable freight trans-

port

This section provides a summary of sustainability problems encountered in urban freight transport. The increasing social concern for the environment and sustain- able growth, in general, requires the transformation of cities. In this context, urban freight transport problems have been analyzed cross-referencing impacts on sus- tainability. As mentioned previously, operations research methods contribute to sustainable management and address usually the logistic issues from a city. In this context, designing a sustainable distribution system by means of vehicle routing

16 Introduction models is the most relevant task for this thesis (Lin et al.,2014).

Another important aspect is the use of electric vehicles which are integrated as an alternative to reduce the environmental impacts generated by freight transport. The inclusion of this transport means dramatically change the urban freight distribution because of the poor infrastructures for alternative fuel vehicles. Accordingly, the main challenge of electric vehicles routing is related to the battery life. In this con- text, this thesis focuses on solving the existing transport problems by the inclusion of the sustainability indicators and electric vehicles.

In practice, the transport field has faced the VRP ever since the vehicles were introduced to meet the needs mobility-related. Consequently, the classical VRP fo- cuses on specific constraints imposed by the involved resources in the distribution process. Nowadays, VRPs not only include the classical constraints but new speci- fications given by the stakeholders or by the new technology advances. Thus, new constraints and attributes set the rich VRPs aimed at sustainable city logistics.

The Multi-depot VRP is an important variant of VRP that represents a distri- bution network with several depots from which it can serve its customers. From the tactical and strategic viewpoint, a variant of the MDVRP is the first problem to be solved. Since the classical objective of the problem is to minimize the number of vehicles and travel distance, here sustainable MDVRP is solved. In order to address and solve the rich VRPs in a practical way, sustainable MDVRP is fragmented into various CVRPs. As a result, the following sustainable VRPs problems have been studied in this thesis:

• The sustainable multi-depot vehicle routing problem is about how the companies decide to make a sustainable freight distribution in urban or rural zones. Usually, a transport company has multiple depots from which their vehicles depart and arrive, and has multiple customers being served from the different depots. Generally speaking, urban freight transport is becoming increasingly complex due to an increase in the number of journeys, and the as- sociated volume and frequency. Transport activities have a significant negative impact on the environment and population welfare, which motivates decision- makers to study the transport efficiency from a sustainability perspective. Consequently, the challenge is to make a route for each vehicle individually so that the vehicles drive in a sustainable way.

• The electric vehicle routing problem with stochastic travel time is focused on the integration of automated vehicles for the freight distribution. The automated driving system on the vehicle performs itself all driving task and monitor the driver conditions. According to The National Highway Traffic Safety Administration the automated vehicles could reduce the frequency of crashes by eliminating some human error on the roads, improving the safety on

1.5 Overview of the thesis 17