## 5 Results
### Vehicle Response to the Pavement Surface State
The corresponding peak of the DLC characteristic is marked with a dotted line in Figure 7. The conversion of PS-3 characteristics from the lowest position at 2.5-5.5 Hz to the highest at 10-18 Hz (Figure 8b) shows possible erroneous driver's sensations about comfortable enough driving, but an imperceptible decrease in road holding. The characteristics of 50 km/h and 70 km/h do not have distinguished peaks or rises (Figure 8c,d); however, the two characteristics have the same feature of higher PS-2 excitation in the middle-frequency range, i.e., 4-9 Hz for 50 km/h and 5.5-12.5 Hz for 70 km/h. The difference between these two cases is that excitation corresponding to 50 km/h affects the human body, while excitation corresponding to 70 km/h additionally affects USM dynamics (wheel contact). The latter is also recognised as a DLC characteristic (marked with the continuous line in Figure 7). The excitation at a low-frequency range of 1-2 Hz sensitive for driving comfort is higher on the PS-3 state, but this is reasonable because it is the worst pavement quality. Figure 8: Roughness representation in the temporal frequency: (**a**) 10 km/h; (**b**) 30 km/h; (**c**) 50 km/h; (**d**) 70 km/h. In general, all PSD curves of pavement excitation tend to descend with each increase (Figure 8). This may lead to a false perception that a higher speed on the gravel pavement gives better driving comfort or safety. The individual analysis of different operating frequencies made it possible to distinguish vehicle stability, comfort, and human body sense. Lower pavement excitation for specific frequencies causes better comfort, but the individual cases of a high DLC warn about the potential of vehicle stability loss. Natural frequencies utilising the quarter car model's damping ratios (Golden car parameters [39]) were determined from the Bode plot (Figure 9a). Natural frequencies were placed into the speed-wavelength-frequency characteristic (Figure 9b), showing sensitive frequencies overlapping pavement wavelengths at a specific driving speed. The characteristic shows that SM (vehicle body) vibrations are amplified by the pavement with the dominant wavelength of 2-30 m at a driving speed in the range of 10-130 km/h, while USM vibrations (wheels and axles) are amplified by the wavelengths of 0.25-3.5 m in length (Figure 9b). For example, normal highway asphalt pavement with an unevenness of 10 m wavelength causes uncomfortable driving at around 30-50 km/h (speed curves cross the solid black line in Figure 9b). However, this speed is not characteristic of highway driving, and thus a hardly deteriorated pavement prevents discomfort. Vehicle instability caused by wheel resonance under such conditions is barely activated as the natural frequency of the USM (wheel hop) does not cross any speed curve at the above mentioned 10 m wavelength. On the contrary, the second example presents unpaved roads with gravel pavement where short wavelengths dominate. Considering the 1 m wavelength, the natural frequency of SM is not reached (solid line in Figure 9b). However, the USM resonances at a speed range of 30-50 km/h (crosses dotted line), thus, causing unstable wheel contact with the pavement. Shorter wavelengths activate USM resonance at a lower driving speed. Furthermore, the human body sensitive frequency range of 4-8 Hz falls into the short-middle wavelength band (0.3-5 m) for a wide range of speeds. The obtained vibrations correspond to uncomfortable and unstable driving at a specific road pavement waviness for a certain speed. In this Section, it was proven that the IRI index could be used to evaluate gravel road conditions; the significance of the proposal for calculating the index using the specific velocity interval from 30 to 45 km/h for gravel pavement was indicated. The PSD and  
Figure 9: Sensitive vehicle frequencies of pavement wavelengths: (**a**) Natural frequencies and damping ratios of the Bode plot; (**b**) wavelength frequency characteristics conforming to driving speed.