Could the same technology be used to disable a vehicle being pursued by police?  (Or, would smart crook would find a way to disable the device.)

Could it be used to monitor someone’s travel route and time?  (Wouldn’t a search warrant normally be required?)

Could a stalker or angry ex-spouse use the technology to pursue their victim?  (Who would be liable for the unintended consequences?  Wrongful death?  Domestic Violence?)

What’s to stop someone from hacking the system and causing havoc on the highway by sending out false messages to other vehicles?  (The jerks that get their kicks writing a computer virus will have fun with this stuff.)

The traffic accident in the fog scenario can be solved by low-power active radar, without opening the civil-rights can of worms.  Something similar could even be tailored to alert a driver that dozed off and is headed off the road.  (Of course, the traffic accident in the fog can be prevented by drivers that have the ability to lift their foot off the gas pedal.) 

Personally, I am convinced that technology is running far ahead of the social and legal framework.  I see a major civil-rights law suit ahead for this technology.  (George Orwell saw it coming.)