One way I could think to implement it without any tracking or data connection connection with no data being transmitted from the vehicle would be by placing infrared strobe lights periodically along the road, possibly at the same places we already have speed limit signs. The flashing is invisible to the human eye but could be picked up by cameras on the vehicle, vary the speed or pattern of the strobe to indicate a different speed limit.
Something pretty similar is already used by a lot of emergency vehicles to trigger green lights, just the arrangement is reversed with a strobe on the vehicle and a sensor on the traffic signal.
Of course such a system would potentially be vulnerable to things like power outages (strobe can't strobe if it doesn't have power) bad weather (heavy fog, or if the camera and/orr strobe are covered in snow,) and someone could potentially circumvent it by just mounting a strobe light on their car pointed at the camera.
You could probably address the snow/fog issue by locking the car to a lower speed if no strobe is detected, maybe 25 or 35mph, because in those conditions people should generally be driving slower anyway, and then you don't have the expense of needing to put strobes around lower speed areas. And the power issue could be addressed with the kind of solar panels and/or backup batteries that already power some streetlights and such.
And for those who tamper with the system to circumvent it, we're never going to stop speeders entirely, but we can increase the fines to make up for lost revenue to keep police departments happy, they make less traffic stops and rake in the same amount of money.