Popa is correct. It seems to me that from a halachic standpoint it is more problematic for a married woman to be alone with a man in a car than for an unmarried woman. If, and it’s a big if, yichud applies the absolute worst thing that could happen to an unmarried woman is that there might be a saffek qidushin requiring a get. A married woman might actually asser herself to her husband.
That said, there are also real considerations of safety. Just because someone is wearing the levush doesn’t mean he’s OK. Bottom line, don’t pick up hitch hikers unless you know them personally and, even then, execise care.
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