Moreover, the [PERSON] guideline [12] incorporates a response spectra method based on Monte Carlo simulations, but it assumes that the mean step frequency of the pedestrian stream is in resonance with the footbridge dominant mode. Finally, it is worth mentioning the more elaborate models that also introduce the human-human interaction as part of the HSI phenomenon. For example, [PERSON] et al. [33] propose a microscopic model of the crowd, with each individual modelled as an \"inverted pendulum\", inspired by the work of [PERSON] [34], [PERSON] [14] and [PERSON] [35]. Then, the individual movement as a member of the crowd is modelled using the concept of \"social forces\" [36], which determine the pedestrian position. A similar approach is presented by [PERSON] et al. [37], but simulating the pedestrian as a spring-dashpot-mass model. The main advantage of these models is that they accurately reproduce the HSI phenomenon, as they consider most of the multiple factors that intervene. Nevertheless, they are complex and cannot be easily applied yet. In addition, as the authors have noted [14; 31], the inverted pendulum equations are very sensitive to the balance law selected, and there is no general agreement about the value of the parameters of the spring-dashpot-mass model that represent the pedestrians [38]. Therefore, it can be concluded that, in spite of the research of the last decades, common criteria for the lateral assessment of footbridges have not been accepted yet. While deterministic models are the more popular ones, they show a great scatter of the effective number of pedestrians given by different guides and consider the HSI phenomenon only partially. Similarly, spectral methods represent better the pedestrian load and the HSI phenomenon, but they are barely represented in the design guidelines. Finally, the more elaborate models are complex and not available in practical engineering applications yet. A footbridge experiences different load scenarios in its overall life cycle. The comfort level, or structural acceleration response, that users are willing to accept for a specific traffic scenario depends on the probability of its occurrence. Controlling the vibration response usually involves increasing the structural damping by external devices, which has an additional cost. The main purpose of this paper is to propose a simple but general formulation, useful for practical engineering applications, to evaluate the lateral vibration serviceability limit state of slender footbridges, even when the crowd density exceeds the \"critical number\" (postlock-in behaviour). This tool allows informed decisions to be made about the use of external devices to control the vibration response, or whether not to do anything if such crowds will be encountered only rarely. The key finding of the study is that the proposed method, based on the frequency domain analysis, allows the response due to the auto-induced component of the pedestrian load to be expressed in terms of an amplification ratio of the response with no interaction. The ratio only depends on the dynamic properties of the structure, making it very simple to estimate the dynamic response and the structural damping required for any possible pedestrian traffic scenario, as well as to analyse the footbridge sensitivity to the HSI phenomenon. The paper continues with the description of the proposed formulation in Section 2. Then, the practical application of the method is presented in Section 3. Finally, the main conclusions and some future works are given in Section 4.