I want my vehicle to keep moving left/right while the buttons are held down?

I want to make it so that my vehicle can move left and right more than just once (currently, it only moves once each time the button is hit. I want it to keep moving while the button is held down, to make the movement more fluid). Here’s my current script for the vehicle (C#):

using UnityEngine;
using System.Collections;

public class playerScript : MonoBehaviour {
	float vehicleSpeedLeftRight = 10f;
	float vehicleSpeed = 10f;
	// Use this for initialization
	void Start () {
	// Update is called once per frame
	void Update () {
		if (Input.GetButtonDown ("Horizontal") )
		transform.Translate( vehicleSpeedLeftRight * Time.deltaTime * Input.GetAxis( "Horizontal" ), 0, 0 );
	void FixedUpdate()
		rigidbody.AddForce(-transform.forward * vehicleSpeed, ForceMode.VelocityChange);
    	rigidbody.velocity = -transform.forward * vehicleSpeed;

(The reason there’s a negative sign next to the transform.forward is because for some reason it was going backwards instead of forwards, so I used that to make it un-inverted)

Input.GetButtonDown only passes true when you press down the button. Input.GetButton passes true the entire time you hold down the button.