Disclaimer: I am not sold on suicide but am entertaining the possibility.
I don't know how to annotate the map but I have posted previously that IMO the most likely route would have been east on 236th (perhaps of interest: this would have passed close to her house, which is also close to a large heavily wooded park. Perhaps one of these was her original destination), then south on Cedar. This route has sidewalks the entire way The sidewalk along Cedar stops at 244th.
You may be wondering why she didn't choose to stop where Lyon Creek passes under 236th. That immediate area is very populated with homes and apartments, and there is a lot of car traffic there at rush hour.
You can "walk" this route yourself via Google maps street view. It is interesting. That's how I verified the sidewalks' existence, which I had recalled incorrectly.
I feel that if she did walk to the culvert, she may have started out with no definite decision or plan solidified yet. But the culvert where she ended up is, I believe, the closest relatively secluded one to her car.