The exercise wants you to forward shift the past perfect continuous to the past continuous.
I still can't agree with you.
The task is to convert it to the direct speech.
When the police officer reported about the accident, it had already happened.
---------driving-------------accident-------------report------------now--------------> t
That is why in my opinion we should use some kind of past perfect as we are talking about something happened before some other time in the past.
And "for some time" suggests continuous. Hence, the past perfect continous.
Just shifting mechanically will not work here, I believe.