Creating a sandbox style traffic system in Unity is a challenge. As our previous posts have shown agents can use NavMesh and calculate shortest paths but the hit is high on processing which in turn limits the number of agents in a scene. Sander van der Vegte is a multidisciplinary game developer who for the past 12 years has fortunate enough to make a living creating games, part of his current development process is a city traffic game and thus finding ways for natural behaviour while maintaining a low processing requirment.
Within the development the user is able to click on a car to take it over while falling off the edge triggers the random camera again.
It is well worth heading over to http://sandervandervegte.nl/ to view his other projects and to keep up to date on City Traffic.