Can the sentence: ''She enjoys driving fast but when she does so her passengers get frightened'' mean (more or less) '' the faster she drives the more frightened her passengers become''?
No, the two sentences mean two different things. First one means that when she drives fast, her passengers start getting frightened. Second means that when she drives faster, her passengers get MORE frightened, so her passengers are all ready frightened when she is driving (but get more frightened when she drives faster).