I've read the media articles about the vigil, and have found something interesting. Maybe I'm thinking too hard (most likely), but
this article states:
Silent Witness Sgt. Jamie Rothschild announced during the event that the department will be putting up billboards around the Valley with Feldman's photo and information about the case. They've yet to determine how many billboards there will be or how long they will stand, but
they will most likely be along I-17, he said. (Bolding is mine.)
I-17 is a freeway that's nowhere near where Allison was killed. Allison's house is in south Scottsdale on the
east side of town - the nearest freeways are the Loop 101 and the Loop 202. I-17 is on the
west side of town. In order to get to it from where she lived, you have to go south on the 101, connect to the 202 west, connect to I-10 and then finally, you'll be able to get to I-17. It's quite a drive. If that's really where they're planning on concentrating the billboards, they really may not suspect someone from her neighborhood killed her.
Now, playing devil's advocate (with myself, ha), there are no billboards located along the Loop 101, and not many are located along the Loop 202. However, there are digital road signs that were in play during the recent Phoenix freeway shootings and they're used during Amber Alerts, traffic alerts, etc. Even so, there are a lot of billboards on I-10, which is more heavily travelled than I-17 and goes in more directions - you can take I-10 all the way through Phoenix coming and going from California and head down south to Tucson where Allison went to university. I-10 has more WAY more visibility than I-17, and it's a freeway that's closer to where Allison lived.
All this rambling boils down to me wondering why they'd put billboards along a lesser travelled freeway that's on the opposite side of town.