Place 30-50 temorary high resolution video cameras along the main arteries of the interstates. This will allow the vehicle to be caught on tape and might give a hint as to :
1) Owner, from the license plate
2) Exact vehicle description so people can keep a look out for it.
3) Possible identification of this twisted killer.
I'm sure that security camera companies will have a good Christmas. They're probably working overtime. Heck, it could be a security camera company drumming up business.
Something else could be done. Put several microphones alongside the security cameras with some software that listens for the signature of a gunshot, localizes it, and points the camera. I believe a system was invented over a decade ago that can precisely locate a gunshot with several mikes. Was invented for this very kind of situation. With current computer technology, Ronco could sell these for $19.95.