Prefabs position gets changed

I am using this code to instantiate a prefab but after some cycles of code the position where prefab gets instantiate changes.This causes overlapping of prefabs. Like for first few x,y =1,1 later its different. I just want to move my prefab on z axis.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 
 public class MoveScript : MonoBehaviour {
 public float objectSpeed = -0.5f;
 public float posY = 0.0f;
 public float posX = 0.0f;
 public Transform prefab;
 public LinkedList<Transform> roads = new LinkedList<Transform>();
 
 private int numberOfRoads = 3;
 
 void Start()
 {
  for(int i=0;i<numberOfRoads;i++)
 {
  Transform road = Instantiate(prefab) as Transform;
 road.Translate(posX, posY, i* road.localScale.z);
 roads.AddLast(road);
 }
 }
 
 void Update()
 {
  Transform firstRoad = roads.First.Value;
  Transform lastRoad  = roads.Last.Value;
 
  if(firstRoad.localPosition.z < -5f)
 {
  roads.Remove(firstRoad);
  Destroy(firstRoad.gameObject);
  
 Transform newRoad = Instantiate(prefab, new Vector3(posX,posY,lastRoad.localPosition.z + lastRoad.localScale.z), Quaternion.identity)as Transform;
 roads.AddLast(newRoad);
 
 }
 
  foreach(Transform road in roads){
  road.Translate(0,0,-8f*Time.deltaTime);
 }
 }
 }