In the case of JM - I assume you are speaking about the Hannah Graham case - I believe we were dealing with an experienced criminal. By experienced I mean he had assaulted women before. He was in the process of choking his 2005 rape victim when someone came along and scared him off. He killed Morgan Harrington and put her body in a place where he could get to and not be seen from the road and her body wouldn't be found for a while. Same with Hannah Graham. In short, he already knew about these places in advance.
In this case, I don't believe that if Christina is no longer with us that it was premeditated. (I know you weren't comparing JM to EA, but I am using it since it was brought up.) If EA is the one, he is not experienced at this. He had no plan. He would probably believe he has very little time. If he is parked on the shoulder he either hides whatever he has to and gets back to his car as soon as possible. The evidence or Christina may not be far from the crime scene. Maybe only yards away. He has to do it in the dark.
I think if he committed the crime it isn't far from where he lives. It's the area where he would have committed the crime and it is the area he probably knows best. He would be a nervous wreck for the 4 days until she is reported missing. Then he has to wonder: What did I leave behind at the scene? What if it is easier to find in the day time than at night? I wish I had known I would have had more time to hide evidence. Assuming he did it, of course.
I hunted and fished for years by the time I was 23 years old. I knew of places where I could hide evidence and it wouldn't be found for some time. I haven't hunted in years and live in another area. I try to think where I would hide a body now and even with a calm mind and plenty of time to plan I'm at a loss. I want some place I can get to in without a 4-wheel drive and not be seen from the road. I don't want houses nearby where I might be seen or heard. I want someplace I can get to in an hour before the sun comes up. I can't come up with good places. And it isn't like we don't have rural areas near my city. And I ride miles of them on bicycle - daily in the spring thru the summer.
If EA did this, he has made many mistakes and I believe LE has uncovered some that they haven't told us about yet. JM was an experienced predator and he got caught. Whoever did this will be caught as well.