Is it correct? ? We don't know, because you haven't given us the context or a full sentence.
We use the past continuous to give background information to an event. It describes the situation or the activity that was in progress at the moment when something else happened.
If your sentence goes something like this -
Now, they were standing at the side of the road just at the moment when a gigantic lorry crashed into a tree.
- then yes, it's the correct tense.