Answer
Forced resonance and underdamping are factors.
Work Step by Step
Forced resonance is a factor because hitting the regularly-spaced bumps in the road, while moving at constant speed, applies a periodic external force upon the car. In other words, the forcing occurs at a particular frequency.
If the driving frequency matches a natural, or resonant, frequency of the system, we have a condition of “forced resonance”.
Suppose that the frequency at which the car hits the bumps matches the natural frequency of the car oscillating vertically upon its springs. If the springs are underdamped, then oscillations resulting from each bump carry on to the next bump, i.e., the oscillations increase in amplitude, and the car shakes strongly. (If the springs were critically damped, or if they were overdamped, the oscillations would diminish greatly before the next bump.)
