How to set max speed for this car ?

I made a code that applies a relative force to a rigidbody, I Calculate it's speed but it keeps increasing as long as I'm pressing the key, I want to add a MaxSpeed to this car so for example when it reaches 120, it keeps running on the same speed until I stop pressing the key, this is my code.

 function Update () {
 
 var enginePower = 99000;
 var brakePower = 40000;
 var speed = rigidbody.velocity.magnitude;
 var maxSpeed = 120;
 
 if(Input.GetKey(KeyCode.UpArrow)){
 gameObject.rigidbody.AddRelativeForce(0,0,enginePower);
 };
 
 if(Input.GetKey(KeyCode.DownArrow)){
 gameObject.rigidbody.AddRelativeForce(0,0,-brakePower);
 };
 
 Debug.Log(speed);
 
 }