I did this one on three chicks the other day and got two very good responses.

"Excuse me" (get's their attention...can be any girl walking, I don't agree that this is "apologizing" because it's so common and natural)

"this is really random" (anticipates that they may think "gee that's random" which creates rapport)

"but I wanted to get a female perspective" (this gives a good reason as to why you stopped her)

"would you mind giving me your opinion on something?" (get's her to commit to answering your question...and gives permission so she can't back away...women usually like giving their opinion anyways)

"would you rather experience an instantaneous connection with someone or would you rather experience that same connection gradually over a long time?"
(this question gives her a choice...hence her opinion)

I like this question because I am familiar with the "connection" theme.

In fact...you can substitute the last question with pretty much anything that may elicit an opinion from her. Like that PUA video on youtube where I heard this funny question that went like this..."My sister was thinking about getting a tattoo of her boyfriends name on her arm...his name is Butch. Do you think that's a smart idea?"

LoL