Unity : Moving object by drag : constant speed, deceleration, and acceleration and deceleration

CarControllerTouch.cs

using UnityEngine;
using System.Collections;

public class CarControllerTouch : MonoBehaviour
{

    float speed = 10f;
    Vector3 destination;
    float rotSpeed = 10;

    public float smoothTime = 0.3F;
    private Vector3 velocity = Vector3.zero;

    void Start()
    {
        destination = transform.position;
    }

    void Update()
    {

        if (Input.touchCount > 0)
        {
            Touch touch = Input.GetTouch(0);

            Ray ray = Camera.main.ScreenPointToRay(touch.position);
            Debug.DrawRay(ray.origin, ray.direction * 100, Color.green);

            RaycastHit hit;

            if (Physics.Raycast(ray, out hit, 100))
            {
                destination = hit.point;
            }
        }

        destination.y = transform.position.y;

        //transform.position= Vector3.MoveTowards(transform.position, destination, speed*Time.deltaTime);
        //transform.position = Vector3.Lerp(transform.position, destination, speed / 5 * Time.deltaTime);
        transform.position = Vector3.SmoothDamp(transform.position, destination, ref velocity, smoothTime);

        //transform.LookAt (destination);

        Vector3 lookVector = destination - transform.position;

        if (lookVector != Vector3.zero)
        {
            Quaternion targetRot = Quaternion.LookRotation(lookVector);
            transform.rotation = Quaternion.Slerp(transform.rotation, targetRot, rotSpeed * Time.deltaTime);
        }


    }
}