A ball with diameter 8 inches starts at point A to roll along the track shown. The track is comprised of 3 semicircular arcs, all concave up, whose radii are $R_1=150$ inches, $R_2=100$ inches, and $R_3=70$ inches, respectively. The ball always remains in contact with the track and does not slip. What is the distance the center of the ball travels over the course from A to B?