Transponders in cars that work in a street grid of sensors. The street grid is set to the local speed limit. The car transponders sense the car speed , warns the driver for 2 seconds. Then fines the cars registered owner. The compliance rates would be so strong that you can get rid of all the expensive traffic calming obstructions. You can get police onto other duties. There are so many chips in modern cars, the hardware is nothing new. If compliance (speed reduction) is high enough , the speed limits could even be raised on some roads.
The big advantage of this is that it is transferable to any locale that you want to set up the grid infrastructure.  Regarding privacy,, Dont speed and you don't register on the network.