"The high-traffic road was dangerous."
You don't say that. You can say 多くの車（が通る道は危なかったです。） That raises the question why this is so. It seems to be that the adjective 多い loses its connection to 車 there. Cars many in what? There's a lot of ～の・が多い車 ("行楽シーズンに多い車のトラブル" = "common car trouble in the pleasure-trip season") . . .
「車が多い」works as a clause, not as an adjective. Although you could call it an adjectival clause, you can do the same with clauses that are predicated by normal verbs. eg:「車が通る→道」"a road for cars to pass through".
Here is a useful link which also contains examples with i-adjective predicated clauses:
Past tense is conveyed here by 危なかった, present tense of which is 危ない です is not grammatically needed for い adjectives it is added for politeness only. And when it is added for politeness it is always just です if you want to use な adjective or a noun then it will become でした for example here: 車が多い道は危険でした