Wir treffen uns an der Tankstelle, bevor wir auf die Autobahn fahren.

Here uns is part of the verb expression sich treffen.

In this sentence, Wir treffen uns means we are meeting / we are meeting up / we meet each other.

German often uses a reflexive pronoun here:

• ich treffe mich
• du triffst dich
• wir treffen uns

So uns does not literally need to be translated word-for-word as ourselves in natural English. It is just how German commonly says to meet up.

Usually, no—not with the same meaning.

• Wir treffen uns = we are meeting (each other / meeting up)
• Wir treffen jemanden = we meet someone
• Wir treffen etwas can also mean we hit something in some contexts

So treffen by itself normally needs an object:

• Wir treffen Paul. = We’re meeting Paul.

If there is no object and you mean meet up, German normally uses sich treffen:

• Wir treffen uns.

Because an is a natural preposition for being at certain places.

So:

• an der Tankstelle = at the gas station

This is idiomatic German. English uses at, but German does not always match English prepositions exactly.

Compare:

• an der Tankstelle = at the gas station
• bei der Tankstelle = by/near the gas station
• zur Tankstelle = to the gas station

In this sentence, they are meeting at that location, so an der Tankstelle fits.

Because an is one of the two-way prepositions. It can take either:

• dative for location
• accusative for movement toward something

Here the idea is location:

• they are meeting at the gas station

So German uses the dative:

• die Tankstelle → der Tankstelle in dative singular

Compare:

• Wir treffen uns an der Tankstelle. = We meet at the gas station.
• Wir fahren an die Tankstelle. = We drive to the gas station.

Because bevor introduces a subordinate clause, and in German subordinate clauses are separated by a comma.

So this is standard and required:

• Wir treffen uns an der Tankstelle, bevor wir auf die Autobahn fahren.

You should think of bevor like other subordinating conjunctions such as:

• weil
• dass
• wenn
• obwohl

They all normally trigger a comma before the clause.

Because bevor makes the clause subordinate, and in German subordinate clauses the conjugated verb usually goes to the end.

Main clause:

• Wir fahren auf die Autobahn.

Subordinate clause with bevor:

• bevor wir auf die Autobahn fahren

That is one of the most important word-order patterns in German.

Because this sentence describes movement onto the highway, not being already on it.

With auf:

• accusative = movement onto
• dative = location on

So:

• auf die Autobahn fahren = drive onto the highway
• auf der Autobahn fahren = drive on the highway

That is why the sentence says:

• bevor wir auf die Autobahn fahren
  not
• bevor wir auf der Autobahn fahren

Yes, auf often means on, but prepositions do not always match English exactly.

With roads and highways, German commonly uses auf:

• auf die Autobahn fahren = get onto the highway
• auf der Autobahn sein = be on the highway

So even though English says onto the highway or on the highway, German uses auf very naturally here.

It is best to learn auf die Autobahn fahren as a set phrase.

Bevor is used before a clause with a verb.

• bevor wir auf die Autobahn fahren

Vor is used before a noun phrase, not a full clause.

For example:

• vor der Abfahrt = before the departure
• vor dem Essen = before the meal / before eating

So:

• bevor wir auf die Autobahn fahren = before we drive onto the highway
• vor der Fahrt auf die Autobahn = before the drive onto the highway

A useful shortcut:

• bevor
  • clause
• vor
  • noun

Yes.

You can also say:

• Bevor wir auf die Autobahn fahren, treffen wir uns an der Tankstelle.

That means the same thing.

But notice what happens in the main clause after the subordinate clause:

• treffen comes immediately after the comma

That is because in a German main clause, the finite verb still has to be in second position:

• Bevor wir auf die Autobahn fahren, treffen wir uns an der Tankstelle.

Because all German nouns are capitalized.

So:

• die Tankstelle
• die Autobahn

This is a general rule in German, not something special about this sentence. Adjectives, verbs, and most other words are not capitalized unless they begin the sentence or follow another capitalization rule.