Creepy, when you think about the "mission creep" as the article puts it. How long before they can track a single phone from point to point on a whim, rather than with a court order? If you're concerned about tracking, turn your cell phone off when in the car - you shouldn't be talking when you're driving anyway. How will they deal with out-of-state travellers as well? There have already been hassles between Missouri and Illinois over out of state auto purchases and riverboat casinos (in one incident, a riverboat turning around in the Mississippi crossed the court-enforced state line and was issued a citation by the State Patrol - I forget which state in which gambling is legal). Could be interesting if the Illinois and Kansas AG's gets a lot of complaints about possible privacy issues from residents travelling into Missouri on a regular basis (St. Louis and KC are border cities).
Thinking technically, does it matter if only one car in ten has a cell phone? I wouldn't think so - one cell phone moving 20 mph on the interstate out of St. Louis into the burbs should be as telling as 100.
You know, we have this type of feature in the Seattle area right now, without cell phone tracking. This is just another reason why, though I want to live in the St. Louis area, I plan to live on the Illinois side of the river...