“I’m looking for a source one way or the other. Also, if it is OK for a Tiyul, it is probably acceptable for the street as well (as far as the Halacha is concerned).”
I wasn’t differentiating between a tiyul and the street as much as I was differentiating between a skirt that comes just above the knees and a long skirt that may momentarily rise above the knee during the tiyul.
The difference is that when a girl walks around with a knee-length skirt over pants, she looks like she is mainly wearing pants and she stuck a skirt over them. When she is wearing pants under a skirt in case her skirt momentarily goes up, this is not the case (one has the look of pants with skirt over it and one is skirt with pants under it). According to R’ Ellinson’s understanding of the reasons why it is a problem for a girl to wear pants, it would make sense to differentiate between the two.
However, R’Ellinson is not a poseik and his understanding may be inaccurate (I can see a flaw with it).