random object creation

i m creating a bike race game where i have object cube and it is running fine ,now i want to create random bikes or cars on same track (with some movement) on the way of that object so that user can feel he is actually on the road when i do
right now my cube object in z direction i want my random object to move in same track with same direction(z)

// VARS FOR RANDOM OBJECT CREATION

// COPY FROM HERE TO END



  var CubePrefab : GameObject;

   var AmountOfCubes : int;

  var RandomPosx  : int;

    var RandomPosy : int;

    var RandomPosz : int;

 function Start() 
      {

         for(var i=0;i<200;i++)
         {      
        if (AmountOfCubes <=200)
           {

       RandomPosx = Random.Range (20,100);
       RandomPosy = Random.Range (0,0);
       RandomPosz = Random.Range (0,2042);
       SpawnRandomCube();
          }
       }
}
   
 
      
     // Move object using accelerometer variables
    var speed = 1;
    var movey=1;


 function Update () {

         var dir : Vector3 = Vector3.zero;
        // we assume that device is held parallel to the ground
        // and Home button is in the right hand
        
        // remap device acceleration axis to game coordinates:
        //  1) XY plane of the device is mapped onto XZ plane 
        //  2) z plane has fixed value 1 which cause it to move in same direction 
        //but user can move in x direction with the phone acceleration sensor   
        dir.x =  Input.acceleration.x;
        dir.z = movey;
        
        // clamp acceleration vector to unit sphere
        if (dir.sqrMagnitude > 1)
            dir.Normalize();
        
        // Make it move 10 meters per second instead of 10 meters per frame...
       //  dir *= Time.deltaTime;
            
        // Move object
        transform.Translate (dir * speed);
        transform.Translate(0, 0, 1);
}

 function SpawnRandomCube()
{

    transform.position = Vector3 (RandomPosx,RandomPosy,RandomPosz);

    var RandomCube = Instantiate(CubePrefab,transform.position,transform.rotation);


   AmountOfCubes = AmountOfCubes + 1;

}