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);

}

This will do the job:

function FixedUpdate() {
    rigidbody.velocity = Vector3.ClampMagnitude(rigidbody.velocity, 120.0);
}