Some points to think about...
Cost to implement this would be extreme... Also if its broadcasting a signal saying "Speed Limit is xxx here" it would need to do it at the start of the street... So close to the corner of a 50km and 80km road for example. You've just turned from one road onto the other, which is the correct speed limit if you're receiving 2 signals? GPS is another solution, I've got software on my phone which tells me the current speed limit for the road I'm travelling on (Trapster) as well as my current speed. Within 10 meters or so of a changed speed limit sign, it alerts me. However GPS isn't always great quality in all locations... Really cloudy or rainy days lowers accuracy, as does driving through the CBD with lots of tall buildings around.
Sometimes when I'm on a 110km freeway, GPS spears me off to a sidestreet 50m to my left and yells at me that I'm doing 110km/h in a 40km/h school zone...
In this situation, would it send an alert to my "speeding probation officer" telling him to lock me up and throw away the key? How would I prove I wasn't speeding, if the 'evidence' shows that I was. If this is linked to speed limiting the vehicles, would it slam on the brakes for me? down to 40 in a 110 zone? Now though... Next Gen GPS Technologies are being tested to replace the current GPS system which is prone to "hacking" from localised transmissions. So one day the GPS issues I've just mentioned could be null and void. Until then though, I don't think we possess the tech / money to make this work effectively.