Die Fahrbahn ist morgens oft frei, aber am Abend ist sie voller Autos.

Fahrbahn is a feminine noun in German, so its article is die. When German refers back to it, it uses the feminine pronoun sie.

In English, you would say it, but in German pronouns follow grammatical gender, not natural gender. So here sie means it, not she.

Because it comes after ist. After sein, adjectives are used predicatively, and predicative adjectives do not take endings.

So:

• die freie Fahrbahn = the clear roadway
• die Fahrbahn ist frei = the roadway is clear

That is why you get frei, not freie.

No. Here frei means clear, open, or unobstructed. In the context of a road, it means there is little or no traffic in the way.

So Die Fahrbahn ist frei means the roadway is clear, not that it is free of charge.

Morgens means in the mornings or every morning. It is a time adverb.

The -s is part of a common adverb form used with times of day and days:

• morgens = in the mornings
• abends = in the evenings
• nachts = at night
• montags = on Mondays

You can think of morgens as a fixed adverb meaning a repeated time.

Yes, abends would also be correct.

The sentence could also be: Die Fahrbahn ist morgens oft frei, aber abends ist sie voller Autos.

The difference is mostly one of style:

• morgens / abends = adverbs, often with a habitual meaning
• am Abend = a prepositional time phrase meaning in the evening

Mixing them is normal and natural in German.

Am is a contraction of an dem.

So:

• am Abend = an dem Abend

In time expressions, this is very common:

• am Morgen
• am Abend
• am Montag
• am Wochenende

In English, you usually translate am Abend simply as in the evening.

German main clauses normally keep the finite verb in second position.

In the second clause, am Abend comes first. That means the verb ist must come next, and the subject sie follows it:

• aber | am Abend | ist | sie | voller Autos

So this is correct German word order.

You could also say: ..., aber sie ist am Abend voller Autos.

That version is also correct, but it puts the subject first instead of the time expression.

Oft means often. It tells you how frequently the roadway is clear in the morning.

In this sentence, it fits naturally between morgens and frei: morgens oft frei

German adverbs can sometimes move around for emphasis, but this position is very normal and easy to understand.

No. Here voller does not mean more full.

In this sentence, voller is part of a fixed expression meaning full of:

• voller Autos = full of cars

This is a standard and fairly idiomatic German construction, especially in writing. More everyday alternatives are:

• voll mit Autos
• voll von Autos

But voller Autos is perfectly correct and quite natural.

In voller Autos, Autos is in the genitive plural after voller.

For many plural nouns ending in -s, the genitive plural looks exactly the same as the ordinary plural, so Autos does not change form.

There is no article because German often uses a bare plural for nonspecific things:

• voller Autos = full of cars

That works much like English, where you also usually say full of cars, not full of the cars.