Thanks for posting on the NI forums! I think that images from above would be far easier to deal with, because you can set your ROI to be one lane, and watch for cars one at a time. If you filmed from the side of the road, you could get two cars passing at once, or cars passing in opposite directions. There would be a lot more interference filming from the side, and it would require a far more advanced algorithm to get data from the images. From above, you can watch for cars against the road, which should be a relatively easy edge to detect.
Good luck!