Who is fault is at fault when you are turning right and a car behind hits you because they are coming to fast behind you?

Who is fault is at fault when you are turning right and a car behind hits you because they are coming to fast behind you?
Assuming you have signalled in good time that you are turning right,most would agree that the fault lies with the car behind-who should have left enough distance to slow down. This happened to a friend of mine-the other driver lied and said that he did not signal,and changed lanes without warning.He was a convincing liar and won the case.So take nothing for granted-even basic facts can be dsputed.