Use cement or asphalt or other material to create a 20 centimeters lane
bikes can ride easilly. That will be much cheaper compared to restoring the whole road.
The bumpy road should be preserved so it functions as a spontaneous traffic calming design to make cars drive slowly.
The construction quality and material doesn't need to be top quality and the lane can remain for a large time, if it is only used by bikes. For that reason, it should be designed in such a way that smart car drivers will not try to ride on top of it with wheels from one side. One solution may be to build it near the curb. Or in case of narrow roads, in the center, so car wheels will be in both the sides but not on top of it.
This solution could be implemented by municipal authorities or by conscious bike citizens. This teams should go to the place at night or when there is few traffic. Use wooden boards to delimit a space with the desired width, and fill it with some centimeters thick cement to make the floor soft. The cement takes some hours to dry but the boards could be removed before.