SImple solution is to just go into the turn slow. This gives a hint to the car behind you that you might be doing something beyond a left turn, and reaction time for any traffic that you may have to yield for (whether or not they legally have the right-of-way). Once you start turning more it ought to be obvious.

Of course lots of accidents are from obvious situations that could have been avoided, but not doing a legal U-turn isn't going to fix stupid people.