ame no hi ha, eki no mae no douro ga zyuutaisurukoto ga ooi.

Because の is doing noun-to-noun linking several times.

In this sentence:

• 雨の日 = rain day → a rainy day / a day when it rains
• 駅の前 = the front of the station / in front of the station
• 駅の前の道路 = the road in front of the station

So の often works a bit like of, or like an apostrophe-s, or like a way to turn one noun into a modifier for another noun.

You can think of it step by step:

• 雨 → rain
• 雨の日 → a rainy day
• 駅 → station
• 駅の前 → in front of the station
• 駅の前の道路 → the road in front of the station

Japanese often stacks nouns this way.

Yes, literally it is rain's day or rain day, but naturally it means:

• a rainy day
• a day when it rains
• on rainy days depending on context

In Japanese, 雨の日 is a very common expression. It does not necessarily mean the whole day is rainy in a strict sense; it just refers to days when it is raining.

は marks the topic.

So 雨の日は means something like:

• As for rainy days, ...
• On rainy days, ...

The sentence is setting up a condition or situation and then commenting on it.

Structure:

• 雨の日は = speaking about rainy days
• 駅の前の道路が渋滞することが多い = the road in front of the station is often congested / often gets congested

So は is not marking the subject here. It is marking the overall topic or frame.

Because within the comment part of the sentence, 道路 is the thing that does the action/state being described.

• 駅の前の道路が渋滞する = the road in front of the station gets congested

Here, が marks the grammatical subject of 渋滞する.

The full sentence has two layers:

1. 雨の日は → topic/frame
2. 駅の前の道路が渋滞することが多い → what is often the case on rainy days

So it is very normal to have both:

• は for the topic
• が for the subject inside the statement about that topic

It means the road in front of the station.

Break it down:

• 駅 = station
• 前 = front
• 駅の前 = the front of the station / in front of the station
• 駅の前の道路 = the road that is in front of the station

So the second の connects the location phrase 駅の前 to 道路.

This is a very common Japanese pattern:

• 学校の近くの店 = the store near the school
• 家の前の公園 = the park in front of the house

Yes, 渋滞 is a noun, but it can also form a する-verb:

• 渋滞 = traffic jam / congestion
• 渋滞する = to be congested / to get jammed / to have a traffic jam

This is very common in Japanese. Many nouns can combine with する to make verbs.

Examples:

• 勉強する = to study
• 運転する = to drive
• 渋滞する = to become congested

So in this sentence, 道路が渋滞する means the road gets congested.

Verb dictionary form + ことが多い means:

• it often happens that ...
• there are many cases where ...
• more naturally, ... often ...

So:

• 渋滞することが多い = it often gets congested / it is often congested

Here, こと turns the whole action 渋滞する into a noun-like idea: the event of becoming congested.

Then 多い says that this event happens frequently.

So literally it is close to:

• Cases of the road in front of the station getting congested are many on rainy days

But in natural English, we would simply say:

• On rainy days, the road in front of the station is often congested.

Yes, a sentence like this is also possible:

• 雨の日は、駅の前の道路がよく渋滞する。

That also means On rainy days, the road in front of the station often gets congested.

The difference is mainly nuance and style:

• よく + verb = directly says often
• Verb + ことが多い = it is often the case that... / there are many instances of...

ことが多い can sound a little more descriptive or observational. It is a very common pattern in both speech and writing.

Both can be used, but the nuance is slightly different.

• 渋滞する focuses on the event or tendency: gets congested
• 渋滞している focuses on the resulting ongoing state: is congested

In this sentence, 渋滞することが多い means that on rainy days, congestion frequently occurs there.

If you said:

• 雨の日は、駅の前の道路が渋滞していることが多い。

that would mean something like:

• On rainy days, the road in front of the station is often in a congested state.

That is also natural, but the original sentence is a little more general and event-focused.

Grammatically, こと often nominalizes a verb phrase, meaning it turns it into a noun-like unit.

So:

• 渋滞する = to get congested
• 渋滞すること = the fact of getting congested / the event of getting congested

Sometimes textbooks say こと = thing, but that is too simple here. In this sentence, it is better to think of こと as:

• the fact that ...
• the occurrence of ...
• the event of ...

So 渋滞することが多い is not really many things congest. It means there are many occasions when it gets congested.

A good way to see it is:

• 雨の日は、 = As for rainy days,
• 駅の前の道路が = the road in front of the station
• 渋滞することが多い。 = often gets congested / it often happens that it gets congested

So the overall structure is:

Topic + subject + predicate

More literally:

• As for rainy days, cases where the road in front of the station gets congested are many.

Natural English:

• On rainy days, the road in front of the station is often congested.

Yes, very often.

• 駅の前の道路 = the road in front of the station
• 駅前の道路 = the road by the station / the road in front of the station

駅前 is a very common compound meaning in front of the station or the station-front area.

In many real-life situations, Japanese would naturally use 駅前:

• 雨の日は、駅前の道路が渋滞することが多い。

That sounds very natural too.

Here are the standard readings:

• 雨 = あめ
• 日 = ひ
• 雨の日 = あめのひ
• 駅 = えき
• 前 = まえ
• 道路 = どうろ
• 渋滞 = じゅうたい
• 渋滞する = じゅうたいする
• 多い = おおい

So the whole sentence is read:

あめ の ひ は、えき の まえ の どうろ が じゅうたいする こと が おおい。

Yes.

• 雨の日は
• 雨の日には

Both are correct.

The に adds a stronger sense of on or in the case of rainy days. It can sound slightly more explicit.

So:

• 雨の日は、駅の前の道路が渋滞することが多い。
• 雨の日には、駅の前の道路が渋滞することが多い。

Both are natural. The version without に is a little simpler and very common.