Вчера на перекрёстке светофор не работал, поэтому движение было опасным.

На перекрёстке uses the preposition на (often used for open areas/surfaces or locations like на улице, на мосту, на площади, and commonly на перекрёстке).
Перекрёстке is in the prepositional case (locative meaning at/on the intersection): перекрёсток → на перекрёстке.

The verb is past tense and agrees with the subject светофор, which is masculine singular.
So: светофор (m.sg.) → (не) работал.
If the subject were neuter, you’d get работало; feminine: работала; plural: работали.

Не работал is imperfective, so it normally means the traffic light was not functioning / was out of order (for some time), without focusing on a single completed event.
If you wanted a more “single event” feel, you might use something like не сработал (didn’t go off / didn’t function at that moment) or перестал работать (stopped working).

Поэтому means therefore / so, and it often links two clauses in a cause→result relationship.
In this sentence, it connects two parts of a complex sentence, so a comma is used before it:

• Светофор не работал, поэтому движение было опасным.

They express the same logic but from different sides:

• потому что = because (gives the cause clause)
  Example pattern: Движение было опасным, потому что светофор не работал.
• поэтому = therefore/so (introduces the result)
  Pattern used here: Светофор не работал, поэтому движение было опасным.

After быть (to be) in the past/future, Russian commonly puts the predicate adjective in the instrumental case to describe a state/quality:

• движение было опасным = the traffic was dangerous (as a condition)

Опасное (nominative) is possible in some contexts, but instrumental is very standard for “was + adjective” descriptions.

Yes. Опасно is a short-form adverbial/predicative style meaning it was dangerous (more like describing the situation generally).

• движение было опасным = focuses on traffic as a thing having the quality “dangerous”
• движение было опасно = focuses more on the overall situation being dangerous
  Both are natural; the sentence you have is very neutral and common.

Движение is a neuter noun (most nouns ending in -е / -о are neuter).
So the past tense of быть agrees as neuter singular: было.
Compare: он был, она была, оно было, они были.

In this context, движение very commonly means traffic / traffic flow (especially with roads and intersections).
So it’s not about physical movement in general; it’s specifically road traffic.

Word order is flexible. The current order is natural and sets time first:

• Вчера (time) → на перекрёстке (place) → main event.

You could also say, for different emphasis:

• На перекрёстке вчера светофор не работал… (highlights the place first)
• Светофор вчера на перекрёстке не работал… (highlights the traffic light first)

That’s the normal pattern: не + verb.
You can change word order for emphasis, e.g. не работал светофор, which stresses it was the traffic light that wasn’t working (often contrastive).

The correct spelling is перекрёстке (from перекрёсток) and it’s pronounced with ё (roughly yo).
In everyday writing, ё is often replaced by е (перекрестке), but the pronunciation usually stays the same. In learning materials, ё is often kept to help you pronounce and stress words correctly.

No. In the present tense, Russian usually drops “to be”:

• Past: движение было опасным
• Present: движение опасное / движение опасно
• Future: движение будет опасным