I do not think it is unreasonable for law enforcement agencies to designate, during the course of an actual investigation of a specific target, individual cars

I do not think it is unreasonable for law enforcement agencies to designate, during the course of an actual investigation of a specific target, individual cars for GPS surveillance... Okay, so why not require a warrant for clear-cut cases like you suggest here? There's a legal mechanism, a historical precedent, a best-practice way of handling this sort of privacy intrusion. What is the compelling reason for throwing all that away? I'm not generally opposed to things that allow law enforcement to do a better job with fewer resources. Do you honestly imagine that a police department that adopts warrant-less GPS trackers will lay off some of the force it no longer needs for stakeouts and surveillance? I don't. This is a more efficient way of doing surveillance, but it isn't something that will be used to "do a better job with fewer resources." It will be used - it is being used - to cast a wider net, to launch larger fishing expeditions, just like every single piece of surveillance technology and surveillance-related legislation in living memory.