Here is an ImageJ macro to measure distance between two user-drawn segmented lines (polylines) on an image. Shortest distances from points along the shorter line to the longer line are used to calculate the average distance between the two lines (their spline interpolations).
The macro works in ImageJ 2.1.0/1.53c, and may be useful to someone.
The macro prompts user to draw the two segmented lines (example below).
The macro then makes the measurements and displays statistics on the image, and prompts the user to save the values in a user-selected file.