I’m wondering if it is possible to write:
I was standing there for a few minutes when the police cars stopped.
and
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.
"Was standing" and "stood" suggest different meanings. "Was standing" suggests that the action was in progress when the police cars stopped, and the most likely interpretation would be that "a few minutes" refers to the time before the police arrived. "Stood there for a few minutes" suggests the two actions do not overlap - either you stopped standing there when the police cars arrived or began standing there when they arrived - and the latter is definitely the more likely (in contrast if it had been "while the police cars stopped" they would have been necessarily overlapping - presumably the police had very bad brakes).
"Had been standing there" is probably the most proper, if you were standing there prior to the police cars arriving. Though I could imagine people using "was standing" in informal speech.
I'd say I was/had been standing for few minutes WHEN the police arrived (action in progress, interrupted by another yet brief action) or I stood there for few minutes THEN the police arrive (processive sequence of events)
