Not much you can do with idiots on the road. When I am turning left with people behind, I will use a hand signal and turn signals. The JD 8530 & 9330, both have turn signals. When there is no on coming traffic, a couple of hundred feet prior to turning, I will drive the tractor and take up most of the on coming traffic lane, both preventing them from passing as well as forcing them to pay attention that I am serious about making a left turn.
With a much smaller compact tractor, and cars behind, assuming I can, it is much safer to pull off the road on the right side and wait for a safe clear moment. Just because the guy behind you is patient, another car for several vehicles back will always tray to be stupid.