void OnDestroy()
 {
     if (instance == this)
     {
         instance = null;
     }
 }
        // Use this for initialization
        void Awake()
        {
            if (instance != null)
            {
                Destroy(gameObject);
                return;
            }
            instance = this;

            numCarsSlider.onValueChanged.AddListener(NumCarsSliderValueChanged);
            highwaySizeSlider.onValueChanged.AddListener(HighwaySizeSliderValueChanged);
        }