Translation:The big buses are moving in the front, and the small cars are moving in the back.
And you could also put the verb in the first part and omit it in the second part. Omitting a repeating word is very common in Hungarian. Example:
"I go home and you go to the bank." - "Én haza megyek, te pedig a bankba (mész)." Not even the same person, just the same verb. It is omittable.
You can do the same thing in English, while probably not as frequently as in Hungarian (remove the repeated verb) -- but the problem here is that English speakers don't use "move" in this way. Depending on the context, you'd say: The big buses are in the front, and the small cars are in the back.
Yes, moving sounds a bit strange to me, too. It seems to me that traffic is what's moving in English, not individual vehicles. And the verb "halad" does not have a clear translation, either. It is a combination of moving, proceeding, being in motion and making progress. You can also use "menni" (to go) as a synonym.
"A nagy buszok elöl mennek, a kicsi autók pedig hátul."
But "haladnak" has a pronounced sense of an action currently in progress. It is progressive by nature.