I don't think we can know his motivation - in the Fairfax case, he lie in wait, and stalked his prey. In Harrington, and presumably, Graham's case, he probably lured them into his car with the promise of a ride, took them to a remote location, and, well.
He was interrupted in the Fairfax case, which possibly made him evolve his technique to getting his victims into his car, then taking his victims to a remote location where he felt safe. That's usually something that you can see in most serial cases, they evolve and refine their technique to a more-or-less characteristic pattern.
And if it bothers anyone why someone would wonder about these things - understanding his evolution, identifying the mistakes he - and the investigators - made, the opportunities missed to identify him earlier, while they won't help these victims, may help investigators learn from their mistakes, and be better able to identify and capture future perpetrators.
But this distance traveled does suggest to me that it was no mere opportunistic event, but that he traveled there with the specific intent to find someone to stalk and rape - and possibly, to kill. And if you accept *that*, then you have to suppose there were probably other attempts by him in the intervening years, and probably other victims.