I’m wondering if it is possible to write:
I was standing there for a few minutes when the police cars stopped.
I stood there for a few minutes when the police cars stopped.
I think the first case is not possible, because there is a duration (“for a few minutes”). Therefore, past perfect continuous would be better: “I had been standing”.
In the second case we don't know which action takes place first, so past perfect continuous is the better choice here as well.