。
She knew that frozen roads are dangerous.
Literal
She [topic-は] frozen road [subject-が] dangerous [fact-であること] [object-を] knew.
凍った道路 uses a past-tense verb as a prenominal modifier — 'roads that have frozen / frozen roads.' The key structural element is ~であることを知っていた: である is the formal copula (replacing だ), こと nominalizes the whole clause ('the fact that frozen roads are dangerous'), and を marks it as the object of 知る. This formal nominalization pattern is characteristic of written Japanese.