I happened to use an open source traffic simulator which is capable of simulating both purely autonomous and human/autonomous traffic scenarios. It is possible to still support traffic lights for the humans who, for whatever reason, still desire to drive (I bet there will be always be reasons for you to want to drive manually.)
The team at the University of Texas at Austin has done a great job developing this simulator, AIM4. You can check out the videos and the source code here:
http://www.cs.utexas.edu/~aim/