A car travels over a dirt road that contains a series of equally spaced bumps (a so-called "washboard" road). While traveling at a given speed the driver experiences a very jarring ride. When the driver drives at a higher speed, however, the ride gets smoother. That is because

When a car moves faster then it made suspension to oscillate at a faster rate than the previous frequency. The comfort of the car depends upon the natural frequency of suspension. A low frequency can give discomfort while the high frequency can also be harsh. An optimum frequency matching the level of human comfort is desired in the suspension system to absorb the shock.

The ride is perceived as comfortable when the natural frequency is at the level of 60 to 90 cycles per minute.