## 1 Introduction  
There are many definitions of risk in the literature. According to the Society of Risk Analysis [1], risk is <the potential realization of unwanted and negative consequences arising from an event>. By ISO 31000 [2] standards, risk is referred to the <possibility of an effect>. The UK Cabinet Office [3] brings risk back to the \"uncertainty of outcome\". Regarding the systematic terminology of the sector, there are variations in relation both to the scientific sector and according to the needs and goals to be achieved [4]. Instead, what is generally recognized is the centrality of risk whenever it is necessary to solve a problem or plan [5]. In fact, numerous international bodies and organizations have promulgated regulatory guidelines about risk management. ISO 31000 [6] has defined guidelines and principles useful to harmonize the management processes of any risk in both existing and future standards. Similarly, the International Atomic Energy Agency (IAEA) [7], the European Commission (EC) [8], the Health Safety and Executive (HSE) [9] and the World Health Organization (WHO) [10] have defined etymological aspects, standards, and procedures for safeguarding human life in high-risk sectors. In this paper, the focus is on the ex-ante assessment of the risk that characterizes investments in the civil sector. Also in this sector, the European legislative framework acknowledges the key role that risk analysis must play in the economic evaluation of projects. The Guide to the Cost-Benefit Analysis of the European Commission (EC) for the 2014-2020 programming period identifies the following four essential phases to assess the investment risk: 1. sensitivity analysis; 2. qualitative risk analysis; 3. probabilistic risk analysis; 4. possible definition of prevention and/or mitigation actions [11]. EC Regulation 1303/2013 clarifies that risk analysis is needed in situations where the exposure to residual risk, i.e., the risk that remains despite the mitigation strategies undertaken, is substantial. However, it can also be performed because of the dimension of the project or in relation to the accessibility of the data required for the evaluation. On the other hand, there is no guideline giving information on acceptable levels of investment risk [12]. To fill this gap, an innovative protocol is proposed for assessing the acceptability of the investment risk of civil works. The idea is to bring ALARP (As Low As Reasonably Practicable) logic into the procedural frameworks of Cost-Benefit Analysis (CBA). Widely used in the field of security, ALARP logic gives a guide to accepting the risk of loss of life in high-risk sectors (energy, nuclear, oil and gas). According to this approach, a risk is ALARP if it is between the thresholds of acceptability and thresholds of tolerability, or if the costs of reducing it are not disproportionately high in relation to the benefits obtainable [9]. From the ALARP approach, criteria relating to risk acceptability and tolerability thresholds are borrowed, becoming useful in assessing the risk acceptability of the project. In particularly, it was considered that the ALARP principle could represent a general way of thinking whenever the overall goal is a triangular balancing of costs, benefits, and risks. The main novelty of the model concerns the methodology to assess these threshold values. This methodology is based on the joint use of the Capital Asset Pricing Model (CAPM) and statistical tools to estimate the acceptance limit values to both the investment sector and the socio-economic context of the investment project. If regulatory guidelines and sector literature provide indications for estimating investment risk, there are no specific references for stable within which range this risk can be considered acceptable for the investor. In this sense, by introducing the ALAPR logic in the CBA procedural scheme, we provide objective criteria to establish whether the probability of investment failure is unacceptable, ALARP, or widely acceptable for the economic operator.