Possible to edit the movement of NavMeshAgent ?

 This js should work.
 Attach it to the nav mesh agent ( your car ) and it shall serve the purpose.
 To make it smoother play with the nav Agents ( Angular speed value ).
 
 targets is an array of Transforms where you can plugin the waypoints.
 currentTarget can be your 1st way point ( targets[0] , or you can drag in the inspector )
 navigation is your NavMeshAgent.
 
 
 public var currentTarget : Transform;
 public var targets : Transform[];
 public var navigation : NavMeshAgent;
 var incr:int;
  
    
 function Start ()  
 {       
   navigation.destination = targets[i].position;   
 } 
  
  
 
 function Update () 
 {   
     var dist = Vector3.Distance(targets[i].position,transform.position);
     currentTarget = targets[i]; 
     if (dist < 5){     
 
     if (i < targets.Length - 1)
         {   
              i++; //change next target
              incr++;
              navigation.destination = targets[i].position;   
              
          }
     }
 
 }

I am not sure if you still need a answer to this but how about using RotateTowards

method? It works rather smoothly for basic humanoid characters as far as I have noticed, might work with a car as well, although I don't think creating a car AI that way would do any good.