In my experience if the (road side) shoulder of the ditch is not close to the same pitch as the scrape blade...it is only going to take a short pass before the blade/rake is loaded with earth/gravel from the shoulder and along with any accumulated debris just falls/rolls back into the ditch...
This is the problem I had when I borrowed a buddies landscape rake. I might as well have been using my FEL (I tried that too)