You should have let them pass.
They were probably coming in the opposite direction, so the op opted to go off the road rather than hit the semi head on...
Oops. I thought I had typed it out.. Still shaken up about it. The semi was coming up behind the car FAST. Posted speed limit was 30mph and the truck was doing at least 45 or 50mph. Car was turning left and put on the blinkers well in advance so the semi could slow down. Semi decided to ignore the turn signals and pass in the oncoming lane, even though its a no passing zone.