Unity Car tutorial, Wrong way code

Hello, I'm trying to add wrong way check to car tutorial, following the instruction on this forum post http://forum.unity3d.com/threads/16706-And-the-winner-is... i added waypoint to the terrain and here is my code

  trackDir = NextwayPoints.transform.position - CurrentWayPoint.transform.position;
    trackDir.Normalize();
    xx = Vector3.Dot(this.transform.forward, trackDir);
    if ( xx< 0)
    {
        wrongWay = true;
    }
    else
        wrongWay = false;

this script is attached to the car in the tutorial, now it works in perfect way as long as its the straight road on the track but once i enter the u turn and sharp turns the code doesnt work any more ... is there somthing wrong with the code ?