Adding Acceleration to Simple Car Controller

I am currently working on a simple arcade style car game. I looked up a tutorial for a simple controller without the use of wheel colliders. It has a Top Speed variable, but no Acceleration, and I don't know how to implement it. Here is the code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class CarDrive : MonoBehaviour
 {
     private Rigidbody rb;
     public float topSpeed;
     // public float acceleration;
     public float turnSpeed;
     private float currentSpeed;
     public float gravity;
     public GameObject lights;
     public Text speedText;
 
     public bool lightToggle;
     private bool isPressed;
 
     private bool IsPressed
     {
         get { return isPressed; }
         set
         {
             if(value == isPressed)
             {
                 return;
             }
 
             isPressed = value;
 
             if(isPressed)
             {
                 lightToggle = !lightToggle;
                 lights.SetActive(lightToggle);
             }
         }
     }
     
     public float Speed
     {
         get
         {
             return currentSpeed;
         }
     }
     
     void Start()
     {
         rb = GetComponent<Rigidbody>();
     }
 
     private void Update()
     {
         IsPressed = Input.GetKey(KeyCode.L);
 
         var mph = currentSpeed * 2.237f;
         speedText.text = mph.ToString("F0");
     }
 
     void FixedUpdate()
     {
         currentSpeed = rb.velocity.magnitude;
 
         if (Input.GetKey(KeyCode.UpArrow))
         {
             rb.AddRelativeForce(new Vector3(Vector3.forward.x, 0, Vector3.forward.z) * topSpeed);
         }
         else if (Input.GetKey(KeyCode.DownArrow))
         {
             rb.AddRelativeForce(new Vector3(Vector3.forward.x, 0, Vector3.forward.z) * -topSpeed * 0.25f);
         }
 
         Vector3 localVelocity = transform.InverseTransformDirection(rb.velocity);
         localVelocity.x = 0;
         rb.velocity = transform.TransformDirection(localVelocity);
 
         if(rb.velocity.magnitude > topSpeed)
         {
             rb.velocity = rb.velocity.normalized * topSpeed;
         }
 
         if(Input.GetKey(KeyCode.RightArrow))
         {
             rb.AddTorque(Vector3.up * turnSpeed * 10);
         } 
         else if (Input.GetKey(KeyCode.LeftArrow))
         {
             rb.AddTorque(-Vector3.up * turnSpeed * 10);
         }
 
         rb.AddForce(Vector3.down * gravity * 10);
     }
 }
 

I'm sure it's an easy implementation, but I'm just not sure where it could go.