[citation][nom]ms1191[/nom]Prove it was me driving the car.[/citation]
In the US, I believe that would likely be the biggest roadblock to implementation, as speeding tickets and other traffic violations are imposed on the driver, rather than the registered owner, of a vehicle. Verifying the driver at the time of the infraction would be a challenge in the best case scenario, and impossible in the worst case.
But hey, those are the sorts of details with which most governments are happy to figure out after they've invested millions in the technology to produce the tickets (that are just going to get thrown out in court anyway).