**1. Introduction**

Retrofitting a roadway lighting is a process continuously developed in urban environments due to installations aging and technical upgrades. The common example is replacing high pressure sodium fixtures with LED, plasma or induction ones (the last two also belong to gas discharge sources) [1] and involving various hardware modules (e.g., occupancy sensors) enabling lighting control [2]. The main problems being analyzed in the related research were reducing energy consumption, improving illumination quality [3], optimizing investment and maintenance costs [4].

It is worth noting that financial optimization of roadway lighting solutions applies also to such non-trivial cases as road tunnel illumination [5,6].

To support retrofit related optimization, a range of computing approaches are proposed including graph-based modeling of lighting systems [7] or evolutionary algorithms [8].

While the computational, technological and financial aspects of lighting design/retrofit are commonly discussed in the literature, a detailed analysis of an environmental impact of lighting system modernization is rather rarely present in the domain research [7,9,10].

To go beyond the obvious fact that a reduced power usage leads to a decreased GHG footprint, we made both qualitative and quantitative analysis of how using advanced lighting solutions influences a greenhouse gases emission. Moreover, we analyzed economic benefits resulting from a lowered CO2 emission, showing that the retrofit investment costs can be effectively reduced by up to 10%.

Similarly, as for the roadway lighting optimization improving energy efficiency, we distinguished three possible retrofit scopes.


As CO2 and other GHG emissions are linearly dependent on the energy usage, we made a parallel analysis of both. This analysis was based on the real-life retrofit, the details of which are presented in Section 3.

In this work, we propose the way of linking a quantitative analysis of financial aspects of a lighting installation retrofit with considerations on an environmental influence (in terms of CO2 emission) of such a modernization. Thus, an environmental influence can be included quantitatively in a retrofit strategy planning. The goal of this work was threefold: (i) finding an impact of the above three retrofit approaches on the resultant CO2 emissions, in terms of their volumes; (ii) finding which lighting classes [13] contribute the most, in the context of emission reduction, to the final financial gains; and (iii) how the reduced CO2 emissions change the annual costs of a lighting installation performance.

This article is structured as follows. In Section 2, we overview the approaches listed above and present estimations of expected energy savings for each of them. Section 3 contains the case study of the retrofit carried out in Cracow, Poland. Beginning with the presentation of its background, we show how particular strategies (i.e., relamping, adjusting lighting installations, and introducing control capabilities) decrease power usage and thus the GHG emission volumes. The extrapolation of results to the scale of a lighting system of the entire city is also presented. Further, we convert CO2 emission volumes to the costs related to the corresponding emission allowances. We show that those costs can reach 10% of savings achieved thanks to decreased power usage. In Section 4, we discuss obtained results. The final conclusions are contained in Section 5.

### **2. Retrofit Strategies–State of the Art**

In this section, the detailed overview of retrofit approaches enumerated in the previous section are presented. The relevant GHG emission reductions are expressed in terms of power reduction ratios as the GHG emissions can be simply obtained on this basis.
