How to move a Transform on top of a series of Transforms

I have script that instantiates a series of Transforms which is the road a car is supposed to drive on. The problem I'm having is that even if the road consists of equal transforms, with only their z-axis position differing the car object seems to collide with the sides of the cubes instead of moving smoothly on top of them.

The way I generate the road is simply an empty GameObject the following script attached (Where roadPiece is a prefab of a cube):

 void FixedUpdate () {
 Instantiate(roadPiece, transform.position, transform.rotation);

}

The parent then moves around and the road that is shaped looks fine.

The car object moves around with the following script:

 void FixedUpdate () {
     if (Input.GetKey(KeyCode.W)){
         rigidbody.AddRelativeForce(transform.forward*acceleration,ForceMode.Acceleration);
     }
     if (Input.GetKey(KeyCode.S)){
         rigidbody.AddRelativeForce(-transform.forward*deceleration, ForceMode.Acceleration);
     }

}

How do I keep the car object from bumping into the cubes of the road and just move on top of them instead?