A man and a woman alone together in a car is a definite yichud problem.
Here’s a true hitching story that happened to a close relative: He was approached on one of the major shopping avenues by a no-longer-married female neighbor. She asked for a ride home. He said, “Sure, I just have to pick up my wife first.” She said, “Oh, forget it then.”
Perhaps she was really in a hurry and thought waiting for his wife would delay them too much, but it gave my relative a very bad feeling, that had he given her a ride there would have been serious consequences.