kyou wa douro ga konzatsushite iru node, basu yori densha no hou ga hayai ndesu.

The roads are crowded today, so the train is faster than the bus.

Breakdown of kyou wa douro ga konzatsushite iru node, basu yori densha no hou ga hayai ndesu.

• は: topic particle
• です: to be
• 今きょ日う: today
• が: subject particle

Questions & Answers about kyou wa douro ga konzatsushite iru node, basu yori densha no hou ga hayai ndesu.

Why are both は and が used in 今日は道路が混雑しているので?

They are doing different jobs.

• 今日は sets the overall topic: as for today / today
• 道路が marks the grammatical subject inside that topic: the roads

So the structure is roughly:

• 今日は = speaking about today
• 道路が混雑している = the roads are congested

A natural way to think of it is:

• As for today, the roads are congested, so...

This kind of combination is very common in Japanese:

• 私は日本語が好きです。
  • As for me, Japanese is what I like.

So は gives the broader frame, and が marks the thing actually doing the predicate.

Why is it 混雑している instead of just 混雑だ or 混雑する?

混雑 is a noun / する-type word meaning congestion or to be congested.

So:

• 混雑する = to become congested / to be congested
• 混雑している = be in the state of being congested

In this sentence, 混雑している describes the current condition of the roads: they are congested right now.

Why not 混雑だ?

• 混雑 is not usually used as a plain な-adjective here in normal conversation.
• The most natural form is 混雑している.

You will also often hear the more casual everyday version:

• 道路が混んでいる
  • The roads are crowded/congested