Dynamic Speed Limits

Speed limit signs are so 1900s. Why are we all limited to the same speed? Traffic conditions, weather conditions, etc. all affect how fast we should drive in order to get to our destination fastest. Couple that with some concerns over fuel efficiency and the rise of intelligent vehicles (we hope), and we have a growing need for dynamic speed limits. What if the speed limit signs knew how the stop-lights ahead of you are synchronized so that it directs you to a speed to travel in order to "hit all the greens?" How about speed limits that know about traffic troubles ahead, and slow you down in order to smooth out the annoying and gas-guzzling nature of stop-and-go traffic? Perhaps when traffic is light, the speed limit will go up.

In any case, once we have intelligent cars, they'll be taking in this information anyways. Why don't we start building the infrastructure to allow intelligent humans to take advantage of this information?