According to TechnoRide.com (currently housed at PC Magazine), the state of Missouri is working on a plan to monitor your every movement on the road, or lack of movement specifically, by tracking the location of your cell phone. 

The state is positioning it as a tactic to help its state traffic agencies effectively monitor current levels of traffic.  For example, if the state can somehow figure out you're on the highway traveling at 65 MPH, and all of a sudden, you slow down to 30 MPH; the assumption will be that you are caught in traffic.  Then, electronic road signs will broadcast the traffic conditions to drivers that are traveling in your direction, and possibly suggest a detour route.  (Of course this is an overly simplistic scenario.)

Sure, it sounds like a great idea, and I'm all for avoiding traffic.  But, it's a very scary proposition that the government wants to track your location by using your own cell phone signal.  According to this article on CNN, the tracking information (e.g., your location at various points throughout the day) is stored in an encrypted format and is destroyed daily.  But can we be sure?  Also, since there's no legal precedent set here, what happens if a government agency issues a subpoena for all records which contain your location (perhaps on an ongoing basis)?  Could these records then be used against you in a court of law? 

Why can't Missouri use sensors embedded in the roads, like other states?   This is a frightening violation of privacy.