Cars oscillate & frequency??

Question

Gravel roads often develop regularly spaced ridges that are perpendicular to the road. This effect, called washboarding occurs because most cars travel at about the same speed and the springs that connect the wheels to the cars oscillate at about the same frequency. If the ridges are 1.8 m apart and cars travel at about 7 m/s, what is the frequency of the springs' oscillation?

___Hz

Answer 1

f = v / L = (7m/s)/1.8 m = 3.88888889 = 3.9 Hz