How to speed up player character in ENDLESS RUNNER after colliding with powerup??

I'm making a 3D endless runner game inspired by Subway Surfers. Thus, as you can guess, the character is constantly running until death. The thing is, I want the player character to run FASTER after colliding with a powerup, but I don't know how to go about doing this. Could someone help? I've been following a great tutorial series by Game Design HQX on YouTube. https://www.youtube.com/watch?v=s0X4_zT5R_s Here is my script. Please be specific on what I should change/add in it as I am still relatively new to C# scripting. Thank you.

Note: I named my character Joe, hence the script name.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class moveJoe : MonoBehaviour {
     public KeyCode moveL;
     public KeyCode moveR;
     Rigidbody playerRigidbody;
     public int laneNum = 2;             //this indicates what "lane" of the path the character is on (2 meaning middle, 1 meaning left, 3 meaning right)
 
     public string controllocked = "n";  //the character can move to another adjacent lane if he is in the middle lane 
 
 
     public float horizVel = 0;    //the character will remain in the middle lane unless 'A' or 'D' is pressed
 
     // Use this for initialization
     void Start () {
 
         playerRigidbody = GetComponent<Rigidbody>();
        
     }
     
     // Update is called once per frame
     void Update () {
         GetComponent<Rigidbody>().velocity = new Vector3(horizVel, 0, 4);
 
         if((Input.GetKeyDown(moveL))&&(laneNum>1)&& (controllocked == "n"))
         {
             horizVel = -4;
             StartCoroutine(stopSlide());
             laneNum -= 1;
             controllocked = "y";
 
         }
 
         if ((Input.GetKeyDown(moveR))&&(laneNum<3) && (controllocked == "n"))
         {
             horizVel = 4;
             StartCoroutine(stopSlide());
             laneNum += 1;
             controllocked = "y";
         }
     }
 
     void OnCollisionEnter(Collision other)         //this is when the character collides with an incoming car
     {
         if(other.gameObject.tag=="Lethal")
         {
             Destroy(gameObject);
             
         }
     }
     IEnumerator stopSlide()
     {
         yield return new WaitForSeconds(.5f);
         horizVel = 0;
         controllocked = "n";
     }
 
     private void OnTriggerEnter(Collider other)         //the powerup deactivates after the character collides with it 
     {
        if(other.gameObject.tag=="Pick Up")
        {
             other.gameObject.SetActive(false);
             
 
 
         }
     }
 
    
 }