## 4 Conclusions and Future Works  
Given the considerable importance of consumption for urban street lighting in the energy and economic balance for many cities, in this work we focus on the selection of the optimal set of energy efficiency retrofitting interventions on public street lighting systems. Contrarily to classical approaches which optimize \"one-off\" interventions, our methodology defines a plan of retrofitting interventions which exploits the initial budget available for the investment as well as financial or environmental benefits and savings resulting from the implementation of energy efficiency measures. The application of the proposed methodology to a real street lighting system in the city of Bari, Italy, highlights the energy savings and financial benefits obtained by the multi-period planning instead of the classical single-period approach. The contribution of the present research is twofold. First, we advance the existing literature on public street lighting by addressing a new topic, namely the multi-period approach for the development of the optimal plan of energy retrofitting interventions in urban street lighting. Second, we equip the city authorities in charge of energy efficiency and green energy for public lighting systems with a straightforward tool for defining the optimal plan of interventions focusing on the lighting system. In fact, a merit of the developed approach is its generalizability to different types of optimization criteria. Whilst in the current model we choose the total energy savings as the optimization criterion, an interesting development of the research can be to apply the retrofit decision-making model to optimize other indicators commonly used for assessing the energy performance of road lighting, and compare the insights that the model provides in these different scenarios. This study is not without limitations. First, one can observe that input data frequently suffers from estimation uncertainties, and sometimes even very small errors could make the \"optimal\" retrofit solution irrelevant from the financial and environmental convenience perspective. Future research will be devoted to include uncertainty into the analysis. Second, public lighting systems are complex systems where retrofitting interventions produce a variety of consequences, related to economic, energy, environment and quality of life factors. Further research will address this issue by extending the analysis to a multi-objective problem. Third, the proposed approach has been developed from the standpoint of the public party who is in charge of the energy efficiency of lighting systems. Actually, very often, due to lack of expertise in public authorities and budget constraints, such interventions are carried out through innovative forms of collaboration with private parties (e.g., Energy ServiceCompanies, typically called ESCOs), namely through public-private partnerships. Further work may also be done in this direction in order to include the two perspectives (i.e., public and private) in the model, thus providing a tool that is able to support the parties in the negotiation and contract design phases. Finally, one may observe that results and implications are derived from a specific and simple case study (where only LED and EHM are considered), since we assume that different retrofit actions produce additive effects on payoff and cost (i.e., we consider a linearly-constrained linear programming in the model). Actually, this limitation is only apparent, since the model can be easily generalized to complex cases with other objective and constraint functions to deal with the eventual and more complex refurbishment actions that would produce combined non-linear effects on the analyzed street lighting system. Figure 3: The NPVs over time intervals achieved by single- and multi-period retrofit planning for different initial investment budgets: (a) \(b=\ell\) 10,000.00, (b) \(b=\ell\) 30,000.00, and (c) \(b=\ell\) 50,000.00.