Set maximum allowable transform for rigidbody

I'm creating a simple elevator. When the player is inside the elevator, ElevatorTrigger reads as 1.
Then, I would like unity to check that the elevator has not exceeded a certain point relative to its beginning. If both are true, the elevator will continue upwards.

How can I set this so I can find and limit how far the elevator has traveled since its beginning? My current code:

 #pragma strict

 var elevatorSpeed : float = 1;
 function Start () {
 
 }
 
 function FixedUpdate () {
 //check if cube is inside of elevator
     if (ElevatorTrigger.inside == 1){
     //set height of elevator
         if 


         (// what do I check here?)


         //speed
             {
             rigidbody.transform.position.y = rigidbody.transform.position.y + elevatorSpeed/10;
             }
     }
 }