Unity2D C# Randomly Spawn GameObject in an Area over another GameObject

I'm new to Unity2D (Unity 5.0.2f1) and have been searching for a solution which I'm sure is staring me in the face!

I have a game object (essentially a road) like below (DirtTrack1):

I have a spawner which spawns GameObjects (vehicles). I want to spawn those vehicles over this road.

I have tried the following code to do this, essentially trying to spawn the vehicle within the Y-axis area of the road by getting the bottom Y co-ordinate of the road and the top Y co-ordinate, so I get the min and max vertical positions of where I can place the vehicle:

 void FixedUpdate() {

     // Repeat spawning after the period spawn
     // route has finished.
     if (!_inSpawningIteration)
         StartCoroutine (SpawnVehiclePeriodically());
 }

 IEnumerator SpawnVehiclePeriodically()     
 {
     // Get the max and min Y of the road
     float roadY = roadObject.transform.localPosition.y;
     float roadHalfHeight = roadObject.GetComponent<SpriteRenderer>().sprite.bounds.size.y / 2f;
     float roadMaxY = roadY + roadHalfHeight;
     float roadMinY = roadY - roadHalfHeight;

     // Pick a vehicle from the vehicle's array.
     // Then, spawn the vehicle within this Y region
     GameObject vehiclePreFab = _vehicles [random];

     // Randomize the Y position that the vehicle will enter at.
     float randomY = Random.Range (roadMinY, roadMaxY);

     // Set the position and spawn.
     Vector3 newPosition = new Vector3 (Const_RoadItemsPositionX, randomY, 0f);
     GameObject vehicle = (GameObject)GameObject.Instantiate (vehiclePreFab, newPosition, Quaternion.identity);
 }

This does spawn randomly but most times it is always not within the road itself. It is either part on the road or at the outside edge of it.

I can't figure out what I'm doing wrong here but I'm sure it is something very simple!