Beatrice was driving like a very slow lunatic. She went twenty-five miles an hour on the highway and kept drifting across lanes so the other cars honked and swerved and ran into guardrails and stuff.
Does "drift" mean a "slow steady movement from one place to another"? Well, I know that there is a motorsport that is called "drifting", but I think that this is not the case. But I believe that when you are in this motorsport, you drift the car, I suppose.
Before a Hollywood film popularized the motorsport sense of drifting, one would have interpreted it in a more general sense:
[with adverbial of direction] walk slowly, aimlessly, or casually
[with adverbial] move passively, aimlessly, or involuntarily into a certain situation or condition
The drifting described in your example is usually implied that the driver is impaired by either drowsiness (where they might keep nodding off) or caused by drugs, alcohol or illness. . . or texting!
Your understanding of the word drift is correct, and so is your assertion that it is not meant in the same sense as in motorsport. Specifically in this case, it is implied that she was meandering unpredictably between lanes, without indicating her intention or waiting for a suitable opening. Had she used indicators and otherwise driven responsibly, other drivers would have been able to act accordingly and avoid running into guard rails.
The sentence is making a sense like "Beatrice was aimlessly wandering from place to place not knowing her proper destination where to go."
