Unityの床をひたすらループさせる

using UnityEngine;
using System.Collections;

public class FloorLoop : MonoBehaviour {
    public float speed;
    float size = 60;

    void Start () {
        speed = -1;
    }

    void Update () {
        transform.position = new Vector3(transform.position.x, transform.position.y, transform.position.z+speed);

        if (transform.position.z + size < 0) {
            transform.position = new Vector3(0,0,transform.position.z+size*2);
        }
    }
}