If you do end up doing this, I highly recommend that you use a camera with full manual control of ISO, aperture and shutter speed. By using a single setting on all of those values you can ensure that the light levels will be comparable between all of the shots. You could actually use good photo editing software to provide relative measurements of brightness.

I would also suggest that shots be taken from two angles if possible, one from the riders perspective and the other from as aerial a perspective as you can manage to get a feel for the pattern of light being cast.