Assuming that there are N lanes and runners are running in the middle of each lane,
width of each lane = (1/N) [m]turning of first lane = π×(R+(1/N)×(1/2))
turning of second lane = π×(R+((1/N)×(1/2))+(1/N))
you can get the difference in turning lane simply by subtracting turning of first lane from turning of second lane.
If i worked the problem correctly, it should be π×(1/N) meter difference between each lane