I'm really not sure I agree at all that the key to this is figuring out the girls' plans, and why they were where they were.
There's so much speculation trying to understand their route, who they were planning to meet, and whether they were lost. I would posit there's a very good chance that they were "just driving." I'm a few years older than they were, but it was incredibly common as a teenager to just get out and drive. The key was to get out of the house. Where you ended up, if you ended up anywhere at all, was secondary. In the days before social media put you in constant contact with all your friends, just being in the car talking was an activity of its own.
And even "plans" weren't plans the way you think of them today. Often the plan for the night was nothing more than a whiff of a rumor.
I heard So and So was having a party.
Was that tonight or tomorrow night.
Not sure, thought it was tonight.
He lives out near Smith Elementary, right?
No, I think that's Some Other Guy. I think So and So is over by The Mill.
Ok, we'll try that first...do you know where he lives?
I was there once, I'm pretty sure...I might recognize it.
Ok, we'll drive there and see if we see cars.
I mean, that was our evening a LOT of nights. Many times you never found it, and didn't really care. Often you decided on the way to do something else, or that you didn't feel like going. It was such a different world when there wasn't any way to just reach out and find out what and where and when. And gas was cheap.
The point is...obviously knowing their exact route would be helpful in finding out where they might have been picked up. However, this assumption that there was logic around their travels, or they wouldn't have gone out of the way for no good reason...I don't buy it. They were just kids, out in a car, killing time and enjoying each others' company. The alternative being stuck at home.
That's not to say that people shouldn't try to nail down every move. But I just think that there's a common assumption that if we could just figure out who they were going out to see or what they drove out there to do, we'd have the killer. I think it's highly likely that isn't the case.
If I had to take a guess, I would now speculate they were killed by a predator and it was a total crime of opportunity. I think they were seen out and around on pretty deserted roads, and someone pulled them over imitating a cop. I'm not sure what the killer might have had in mind, but it might have gotten out of control. There are many people with cop fetishes that get caught over and over pulling women over.
I think it's possible that this was their first crime of this nature...this feels like something to me that just didn't go according to plan. It doesn't see like a well executed crime. I think it's possible that he imitated a cop, pulled them over, with Penthouse letter fantasies about what they might be willing to do to avoid a ticket. It didn't go the way he expected, maybe there was panic, and things escalated.
I would look at everyone who's been arrested for imitating a cop in all the years since and try to find if there are any with ties back to the area. I think there's a chance it's someone not settled in the area. That's a pretty deserted place for to be just "passing through", but there are any number of reasons why someone could have been in the area and crossed paths with them.