this is pretty complex. If you just started using the car tutorial you will have to learn alot before implementing something like this.
There is a traffic system some guys are developing, just search for traffic
This http://www.youtube.com/watch?v=z4vgc1WAi6U&feature=plcp is probably the most advanced and accurate traffic asset for Unity. This guy is really talented. I talked to him over Skype and he wants 3000 $ … but he also wants to think about making a light version or change the hole price.
However if you want to implement your own system you have to start from scratch because most AI assets are not suitable for this situation.