Instantiate problem, spawning too many clones, then cloning the clones! help!

So i've been trying to make my first 3d project, which is a endless runner, instead of moving the road on the z axis, I've decided to move the player instead.

I have my road, and a box collider/trigger just before the middle. at the end of the road I have a box with no collision, and I turn off mesh rendering. then just after the road I have another trigger.

The first trigger is used to spawn the next road, the box at the end is an anchor point for the next road to spawn at, and the final trigger is to destroy the previous road.

This all worked fine, until it crashed because when instancing the prefab, it clones it multiple times.

at this point, the car is on the second road.....the first was destroyed ( as desired) so there should only be two roads left.......but theres not!

Here is my "road spawn" code, attached to the trigger in the middle of the road.

 public class RoadSpawn : MonoBehaviour {
 
 
     public GameObject Road = GameObject.Find("Road_prefab (1)");
     public GameObject Road_End = GameObject.Find("Cube");
     public GameObject car = GameObject.Find ("car1");
     GameObject Road_prefab;
     int Num = 0;
 
     void OnTriggerEnter (Collider car)
     {
         GameObject Road_prefab_clone = Instantiate (Road, Road_End.transform.position, Quaternion.identity) as GameObject;
         Num++;
         if (Num > 3) {
             Destroy (Road_prefab_clone.gameObject);
         }
     }
 
 
     void Update ()
     {
 
 
     
     }
 }

and here is the code for the destroying trigger after the road.........

     public GameObject Destroyer = GameObject.Find("Road_prefab");
     public GameObject car = GameObject.Find("car1");
     // Use this for initialization
 
     void OnTriggerEnter(Collider car)
     {
         Destroy (transform.parent.gameObject);
         }
 
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }

Can anyone please help?!