Translation:We are not going through in front of the car, in which American policemen are sitting.
Not necessarily. The az here is more of a marker for the word autó. Autó gets marked because the relative clause refers to it. In English you don't need such a marker because you usually place the relative clause right after the item it refers to, but in Hungarian that clause can be wherever (at the end of the main clause, mostly), so in a sentence with multiple nouns you need to make sure it refers to the correct one. A clumsy example:
- Az a madár repül át a busz fölött, amelyik szép. - The bird that is beautiful flies over the bus. (the bird is beautiful)
- A madár a fölött a busz fölött repül át, amelyik szép. - The bird flies over the bus that is beautiful. (the bus is beautiful)
- A madár átrepül a fölött a busz fölött, ami szép. - The bird flies over that bus, which is beautiful. (the situation is beautiful)
Both "the car" and "that car" should be accepted here, depending on how much emphasis you want to give the az.
There isn't mentioned anything you can go "over" in this sentence. I'm not sure you can use it in English like that if neither goal nor path are given. So I can imagine "We go over to the other side in front of the car" or "We go over the street in front of the car" (although that's already a bit wonky), but "We go over in front of the car" sounds pretty weird. "We go across in front of the car" would be much better.
The prefix át- means that you go from one side of something to the other (and end up in a comparable position). English makes a difference between flat objects (streets, borders, rivers, etc.) using "across" and objects with volume (like houses, air, forests, rivers, etc.) using "through". Sometimes English doesn't translate the prefix át- at all, for instance with átül, where you get up from one seat and sit down on another (usually in the same row).
It's a bit difficult to formulate it precisely, but I'm going to assume that the car in question is parked on the side of the street. In that case, you want to go from the street to the sidewalk (or vice versa) by passing in front of the car.
"Going through" seems appropriate if there is another car parked in front of the police car, and you "pass through between" those two cars. You're going from one side (the street) to the other side (the sidewalk) by traversing a distinct zone (the row of parked cars).