That was at onetime called "Freeway Hop".
It can even be dangerous at higher speed.
Usually of you slow down or sometime speed up a little
you will pass the pint where the expansion joints
interact with the wheel base and speed to setup the
harmonic vibration that causes the problem.
I had a sports car with a very tight suspension.
At certain speed it would almost bounce off the pavement.
Go above or below that speed and there was no problem.
I soften the suspension to near stock and the problem went away.
My suspension was too stiff.
