AI car Issue!

Posting the code helps us all debug your problem! :D

 var waypoints : Transform[];
     var waypointRadius : float  = 1.5;
     var damping : float = 0.1;
     var loop : boolean = false;
     var speed : float = 2.0;
     var faceHeading : boolean = true;
  
     private var targetHeading : Vector3;
     private var currentHeading : Vector3;
     private var targetwaypoint : int;
     private var xform : Transform;
     private var useRigidbody : boolean;
     private var rigidmember : Rigidbody;
  
  
     // Use this for initialization
    function Start() {
         xform = transform;
         currentHeading = xform.forward;
         if(waypoints.Length<=0)
         {
             Debug.Log("No waypoints on "+name);
             enabled = false;
         }
         targetwaypoint = 0;
         if(rigidbody!=null)
         {
             useRigidbody = true;
             rigidmember = rigidbody;
         }
         else
         {
             useRigidbody = false;
         }
     }
  
  
     // calculates a new heading
     function FixedUpdate() {
         targetHeading = waypoints[targetwaypoint].position - xform.position;
  
         currentHeading = Vector3.Lerp(currentHeading,targetHeading,damping*Time.deltaTime);
     }
  
     // moves us along current heading
     function Update(){
  
         if(useRigidbody)
             rigidmember.velocity = currentHeading * speed;
         else
             xform.position +=currentHeading * Time.deltaTime * speed;
         if(faceHeading)
             xform.LookAt(xform.position+currentHeading);
  
         if(Vector3.Distance(xform.position,waypoints[targetwaypoint].position)<=waypointRadius)
         {
             targetwaypoint++;
             if(targetwaypoint>=waypoints.Length)
             {
                 targetwaypoint = 0;
                 if(!loop)
                     enabled = false;
             }
         }
     }
  
  
     // draws red line from waypoint to waypoint
     function OnDrawGizmos(){
  
         Gizmos.color = Color.red;
         for(var i : int = 0; i< waypoints.Length;i++)
         {
            var pos : Vector3 = waypoints[i].position;
             if(i>0)
             {
                 var prev : Vector3 = waypoints[i-1].position;
                 Gizmos.DrawLine(prev,pos);
             }
         }
     }