This brought me to the idea of cars in the future all with GPS as a standard feature. When the cars are first registered, the plate number and owner details are recorded in a database/system that also tracks speed, location of vehicle - in case of it being stolen, etc. Of course licence plates can be removed so the plate details would have to be recorded somewhere else on the vehicle. Maybe in a microdot format which is then sprayed in various locations on the car body. As for the speeding side of things, if somehow the GPS was set to allow a maximum speed of say 110km (They may be able to do this already, I only use the GPS on my Nokia 5800) and if any speed over the 110km was then recorded, the tracking system would register it and the owner of the car would be sent a ticket. It would have privacy issues, but if it was set up so only speeding was recorded, and not being able to follow someone's daily driving, e.g. from home to shops, work etc. The only way the vehicle could be "followed" would be if it was stolen. And a system like this would free up traffic officers, remove the need to have roadside speed camera's and other means of catching speeders.
Of course, something along these lines may exist already, but just my thoughts when out travelling for work.