3D "Frogger" traffic respawning incorrectly

In my game, I have a row of cars driving down a road (one is a sedan, one is a jeep.) Each car has an empty parent in the center of it with a rigidbody attached and this script:

 var dir : Vector3;
 var speed : float;
 function Update(){
 transform.position += dir.normalized * speed * Time.deltaTime;
 }
 function OnTriggerEnter(other : Collider){
 if(other.tag == "CarEndpointRight"){
 transform.position = Vector3(-27, transform.position.y, transform.position.z);
 }
 }

Each car is spaced 8m apart. The x position, "-27" is the location of the last car in the line before the traffic starts moving. My problem is, even though I believe I have the number set correctly, each time the cars respawn by entering my "traffic endpoint" with a box collider trigger, the gap between the sedan and the jeep gets smaller and smaller, until they're bumper to bumper. Obviously, this won't do in a game similar to Frogger.

I should also mention the car is changed from sedan, to jeep, to sedan, etc. for every other car. What am I doing wrong?