تصل شرطة المرور بسبب الحادث المروري.

Because Arabic very often uses verb–subject–object/complement order, especially in straightforward statements.

So:

• تصل شرطة المرور ... = Arrives the traffic police ...
  Natural English: The traffic police arrive ...

You could also say:

• شرطة المرور تصل ...

This is also correct, but it feels a bit different in emphasis. Starting with the verb is very normal in Modern Standard Arabic.

Because شرطة is a feminine singular noun in Arabic, even though in English police feels plural.

So Arabic is treating شرطة المرور as the traffic police force / traffic police as one unit.

That is why the verb is:

• تصل = she arrives / it arrives / arrives in form, but here it means the traffic police arrive

Not:

• يصل
• يصلون

The grammar follows the noun شرطة, which is feminine singular.

It is an iḍāfa construction, often called a genitive/construct phrase.

Literally:

• شرطة = police
• المرور = traffic

Together:

• شرطة المرور = traffic police
• literally: the police of traffic

Important things about iḍāfa:

• The first noun usually does not take ال
• The second noun can make the whole phrase definite

So even though شرطة has no ال, the whole phrase شرطة المرور is definite because المرور is definite.

Because it is the second part of the iḍāfa phrase.

In Arabic, in an iḍāfa like this:

• first noun = possessed thing or main noun
• second noun = the thing it is connected to

So:

• شرطة المرور = traffic police

The second noun المرور is definite, and that makes the whole phrase definite.

This is why Arabic does not say:

• الشرطة المرور ❌

In an iḍāfa, the first noun normally does not take الـ.

بسبب means because of, due to, or as a result of.

So:

• بسبب الحادث المروري = because of the traffic accident

Grammatically, بسبب is followed by a noun in the genitive case.

In fully vowelled Arabic, that would be shown on الحادث. In normal writing, the short vowel is usually not written, but the grammar is still there underneath.

Because in Arabic, adjectives usually come after the noun they describe.

So:

• الحادث = the accident
• المروري = traffic-related

Together:

• الحادث المروري = the traffic accident

This is the normal Arabic order:

• noun + adjective

not:

• adjective + noun

Because Arabic adjectives must agree with the noun they describe in several ways, including definiteness.

Since الحادث is definite:

• الحادث = the accident

the adjective must also be definite:

• المروري = the traffic-related

So Arabic says:

• الحادث المروري = the traffic accident

If the noun were indefinite, the adjective would also be indefinite.

Formally, تصل is in the imperfect form, which can express:

• present
• near future
• habitual action
• sometimes a narrative present

Without extra words, it often translates as arrives / are arriving / arrive depending on context.

So here it could mean something like:

• The traffic police arrive because of the traffic accident
• The traffic police are arriving because of the traffic accident

If you wanted to make the future clearer, you could use:

• ستصل شرطة المرور ... = The traffic police will arrive ...

They are different tenses:

• تصل = imperfect
  arrives / is arriving
• وصلت = perfect
  arrived

So:

• تصل شرطة المرور بسبب الحادث المروري = The traffic police arrive / are arriving because of the traffic accident

But:

• وصلت شرطة المرور بسبب الحادث المروري = The traffic police arrived because of the traffic accident

Because المروري is an adjective meaning traffic-related.

Compare:

• شرطة المرور = traffic police
  Here المرور is a noun inside an iḍāfa phrase.

• الحادث المروري = traffic accident
  Here المروري is an adjective describing الحادث.

So Arabic uses two related forms:

• المرور = traffic
• المروري = traffic-related

This is very common in Arabic: one idea may appear once as a noun and elsewhere as an adjective.

A fully vowelled version would be:

• تَصِلُ شُرْطَةُ الْمُرُورِ بِسَبَبِ الْحَادِثِ الْمُرُورِيِّ

This shows the grammatical endings more clearly:

• تَصِلُ = the verb
• شُرْطَةُ = subject, so it takes ـُ
• الْمُرُورِ = second noun in iḍāfa, genitive
• بِسَبَبِ = preposition-like expression
• الْحَادِثِ = genitive after بسبب
• الْمُرُورِيِّ = adjective matching الحادث in case, gender, number, and definiteness

In normal Arabic writing, these short vowels are usually omitted.