"They drove across the southwest of the country."
Translation:Ils ont conduit à travers le sud-ouest du pays.
I am no expert, but...
Ils conduisaient quand un lapin est tombé du ciel = they were driving when a rabbit fell from the sky.
In the case given, however, the action has finished (without any falling rabbits, presumably) at some point in the past; donc, at this point, we (can) use passé composé. "They DROVE across...".
i find imparfait a little tricky but I am starting to get the hang of it. It has been useful for me to use it as Duo does (i.e. "used to (do something)") or in the sense of "while I was doing something (e.g. WAS DRIVING - not DROVE), something else happened (e.g. the rabbit FELL - not WAS FALLING)".
Hope that's useful.