## 5 Summary of Presented Approaches  
To conclude this review, Table 8, summarising the main strengths and drawbacks of the approaches discussed in this paper, is provided. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Hypothesis** & **Method** & **Frequency Range** \\ \hline \multirow{4}{*}{[PERSON] [46; 131]} & Features of the acoustic & \multicolumn{2}{c}{Linear predictive coefficients,} \\  & signal depend on the & mel-frequency cepstrum coefficients & \\  & friction coefficient & and power spectrum coefficients, & Not specified \\  & & Artificial Neural Networks & \\ \hline \multirow{4}{*}{[PERSON] et al. [49]} & Features of the acoustic & \multicolumn{2}{c}{Audio samples 20 Hz-20 kHz,} \\  & signal depend on the & \multicolumn{2}{c}{Support Vector Machines} & \\  & friction coefficient & & Support Vector Machines & \\ \hline \multirow{2}{*}{[PERSON] et al. [47; 132]} & acoustic signal & \multicolumn{2}{c}{Power Spectral Density} & \multicolumn{1}{c}{\textgreater{}} \\  & in the tyre cavity & & \\ \hline \multirow{4}{*}{[PERSON] et al. [77]} & tyre circumferential & \multicolumn{2}{c}{In-tyre accelerometer} \\  & acceleration and & \multicolumn{2}{c}{measures vibrations,} \\  & the road friction potential & \multicolumn{2}{c}{ratio of vibrations at low} \\  & and high frequency range & & and 600–2500 Hz \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of high frequency vibration contributions on friction estimation. The following points are intended to help the selection of the most suited approach for each road friction monitoring application and can be taken as a general guide:  
* Slip slope solutions present high variability and still lack of robustness. Further investigations on a wider range of rigid road surfaces are required to corroborate the proposed slip slope versus friction models. * Lateral or longitudinal model-based approaches require high excitation levels. These can produce accurate estimates during emergency braking manoeuvres or limit cornering events, but cannot anticipate the road grip potential during constant speed conditions. Therefore, such approaches are limited to vehicle systems, such as ABS or ESP, and are not suitable for ADAS functions, such as ACC or AEB. * Active force excitation is a promising and novel methodology with which accurate friction estimates can be obtained during constant speed driving. Nevertheless, this solution might require the use of additional sensors (e.g., reference velocity from GPS). Moreover, additional studies assessing \"when\" the active excitation has to be performed are still necessary (e.g., tyre/brake pad wear versus frequency of active excitations to detect sudden changes in the road friction). * Data-based solutions require the collection of experimental data and the subsequent training with these data. In addition, extrapolation issues might arise if the training data do not cover a wide range of road surfaces. Moreover, a certain level of dynamic excitation is required. * SAT-based approaches are limited to vehicles incorporating an Electric Power Steering (EPS) system. In case of electrohydraulic steering system, additional measurements (e.g., tie rod axial force) are required. Model-based approaches that monitor the pneumatic trail can be affected by the accuracy of the suspension or steering kinematic model employed. Tyre model-less approaches such as Lateral Grip Margin might be more robust to kinematic model uncertainties. In any case, lateral excitation levels of at least 0.3 G are reported in the literature to achieve an accurate friction estimation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  
* Low-frequency vibration-based approaches rely on the slip slope principle. Therefore, their applicability is subjected to the validity of this principle. As mentioned in the first point, the relationship between the tyre stiffness and the road friction potential requires further analyses.