This wouldn't be one of my main lines of enquiry. Every day thousands of people will have driven close to where Melanie was found.
The killer could have gone to the slip road with the planned intention of dumping Melanie there, but he could just as easily have dumped her body there in a panic.
I think you make a very important point here.
It is virtually impossible to distinguish whether the man who deposited Melanie's body did so in a hurry, or had pre-planned the location of the deposition site.
However, I also think it's important to distinguish between 3 key factors;
How long he may have been at the site
How long it took him to choose that particular spot
How long he had spent with Melanie's body post mortem
The first 2 are known only to the killer himself and difficult to decipher.
However, trying to determine how long he may have spent with Melanie post mortem is something that we can reasonably certain about.
He used bin bags and then used rope to tie and secure the body.
That alone would have taken some time.
If Melanie's body had just been dumped without the bin bags and rope, then the killer could have just dumped her body straight from his vehicle after having just murdered her.
However, the killer took some time to almost prepare her body to be deposited.
It's interesting how Melanie wasn't found by a road side with the bin bags or rope, or in a ditch somewhere. Instead the killer chose to conceal her remains rather than just deposit her out in the open for all to see.
The fact that the killer took some time to prepare the body for concealment at a hidden deposition site close to the road, would imply that the killer really didn't want Melanie to be found. This is supported by the head and facial injuries inflicted on Melanie.
IMO, this may suggest that the man who murdered Melanie was known to her in some capacity. Otherwise, the killer has gone to great lengths to conceal the body in hidden undergrowth, yet still felt the need to use bin bags and rope in addition to that.
The big question for me is this;
Did Melanie ever leave the killer's car alive, or was she murdered in the car and then taken straight to the deposition site straight from his car? If the latter, then the bin bags and rope were already in his car, and he had pre-planned to murder someone that night.
But of course, the other option is that the killer didn't drive a car, but instead may have driven a van.
If he drove a van, then he could have hidden Melanie's body in the back and then had the time and private space to murder her, put her in bin bags and then use rope to secure the body, all in the comfort and privacy of his own van.
I find it difficult to believe that the killer murdered Melanie after abducting her in a car, and her not having left the vehicle to be placed in the bin bags and tied with rope, as this would surely have required more physical space to accomplish this.
So the killer either needed a vehicle with space and concealment; like a van, or he had a bolt hole somewhere, that he could then take his time to use the bin bags and rope.
Ultimately, by choosing to use bin bags and roper, the killer gives us certain clues as to his mindset in terms of how long he spent with his victim's body before he then deposits her remains.
There is also a possibility that he took Melanie alive and well to the deposition site, and then murdered her where she was later found. However, this seems very unlikely, because why then use the bin bags and rope, and why take a risk of Melanie being seen on the slip road get out of the car?
IMO, it seems likely that Melanie was abducted by being hit and then thrown into to back of a van. And because a full set of Ford keys were found, it is even more likely that the van was a Ford.
A Ford van possibly being a work van belonging to a builder, roofer, mechanic etc...
Lots to ponder