the complete sentence of "路上很多车" is "路上有很多车". the subject is "路(road )", but "太慢" can't modify for "路"(if no "也很慢", then you're right too), so "车" needs to be put in front. the complete sentence of "路上车很多" is "在路上车很多". the subject is "车". "在路上" is a adverbial. "我不开车" and "因为路上车很多…" are two complete sentences. if need to connect them to one, you need to add a connecting verb "是". else divide them with a comma. "我不开车，因为路上车很多…". in addition, you can say "我不开车因为路上车很多…"in spoken.

Perfect