How to set reverse speed limit of car?

i wanna set reverse speed limit of my car. here is my car controller script.

using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityStandardAssets.CrossPlatformInput; using UnityEngine.UI;

public class controlls : MonoBehaviour {

 public Rigidbody rb;
 public WheelCollider FR,FL,RR,RL;
 public Transform wheelFR, WheelFL, WheelRR, WheelRL;
 public Transform com;
 public float trunRadius = 16f;
 public float torque = 25f;
 public float brakeT = 100f;
 public float Antiroll = 2000.0f;
 public Text speedText;

 public Camera cam1;
 public Camera cam2;

 void Start () {
     
     rb.GetComponent<Rigidbody> ();
     rb.centerOfMass = com.localPosition;

     cam1.enabled = true;
     cam2.enabled = false;
 }

 public float speed()
 {
     return rb.velocity.magnitude * 7f;
 }

 void Update () {
     
     DoRollBar (FR, FL);
     DoRollBar (RR, RL);

     FL.steerAngle = CrossPlatformInputManager.GetAxis("Horizontal") * trunRadius;
     FR.steerAngle = CrossPlatformInputManager.GetAxis("Horizontal") * trunRadius;

     rotateWheelMesh ();

     FL.ConfigureVehicleSubsteps(5f,12,15);
     FR.ConfigureVehicleSubsteps(5f,12,15);
     RR.ConfigureVehicleSubsteps(5f,12,15);
     RL.ConfigureVehicleSubsteps(5f,12,15);

     speedText.text = "Speed: " + speed().ToString("f0") + " Mp/H";

     if (speed () < 110) {
         RR.motorTorque = torque * CrossPlatformInputManager.GetAxis ("Vertical");
         RL.motorTorque = torque * CrossPlatformInputManager.GetAxis ("Vertical");
         FL.motorTorque = torque * CrossPlatformInputManager.GetAxis ("Vertical");
         FR.motorTorque = torque * CrossPlatformInputManager.GetAxis ("Vertical");

     } else {
         RR.motorTorque = 0;
         RL.motorTorque = 0;
         FL.motorTorque = 0;
         FR.motorTorque = 0;
     }
 }
 public void OnPointerDown()
 {
     FL.brakeTorque = brakeT;
     FR.brakeTorque = brakeT;
     RR.brakeTorque = brakeT;
     RL.brakeTorque = brakeT;

     RR.motorTorque = 0;
     RL.motorTorque = 0;
     FL.motorTorque = 0;
     FR.motorTorque = 0;
 }
 public void OnPointerUp()
 {
     FL.brakeTorque = 0;
     FR.brakeTorque = 0;
     RR.brakeTorque = 0;
     RL.brakeTorque = 0;
 }

 public void touchcam()
 {
     cam1.enabled = !cam1.enabled;
     cam2.enabled = !cam2.enabled;
 }

 void rotateWheelMesh(){

     WheelFL.localEulerAngles = new Vector3(WheelFL.localEulerAngles.x, FL.steerAngle - WheelFL.localEulerAngles.z, WheelFL.localEulerAngles.z);
     wheelFR.localEulerAngles = new Vector3(wheelFR.localEulerAngles.x, FR.steerAngle - wheelFR.localEulerAngles.z, wheelFR.localEulerAngles.z);

     WheelFL.Rotate(RL.rpm / 60 * 360 * Time.deltaTime, 0, 0);
     wheelFR.Rotate(FR.rpm / 60 * 360 * Time.deltaTime, 0, 0);
     WheelRL.Rotate(RL.rpm / 60 * 360 * Time.deltaTime, 0, 0);
     WheelRR.Rotate(RR.rpm / 60 * 360 * Time.deltaTime, 0, 0);
 }
 void DoRollBar(WheelCollider wheelL,WheelCollider wheelR)
 {
     WheelHit hit;
     float travelL= 1.0f;
     float travelR = 1.0f;

     bool groundedL = wheelL.GetGroundHit (out hit);
     if (groundedL)
         travelL = (-wheelL.transform.InverseTransformPoint(hit.point).y -wheelL.radius) / wheelL.suspensionDistance;
     
     bool groundedR = wheelL.GetGroundHit (out hit);
     if (groundedR)
         travelL = (-wheelR.transform.InverseTransformPoint(hit.point).y -wheelR.radius) / wheelR.suspensionDistance;

     float antirollforce = (travelL - travelR) * Antiroll;

     if (groundedL)
         rb.AddForceAtPosition (wheelL.transform.up * -antirollforce, wheelL.transform.position);

     if (groundedR)
         rb.AddForceAtPosition (wheelR.transform.up * -antirollforce, wheelR.transform.position);
 }

}