NavMesh: Traffic System

I have made a basic Traffic System with 50 car Objects in my scene, roaming around the city, along a dedicated path(waypoints). setup on a NavMesh. this is for Android Mobile.
Now i would want to implement some basic behaviours for the cars like,
1. sensing for any obstacle ahead, obstacle maybe another cars or any other moving crowd unit.
(here what will be good, Using RayCast, Triggers or calculating distance between cars or any other option).
2. obstacle sensing and then slowing coming to a halt, and then after the obstacle has moved then again
starting again to move.
3. sensing if any other cars is besides while moving and slowing down or speeding up accordingly.

Any Ideas/Suggesitons for it.

 

Can we see it?