Cars driving the wrong way

I'm making a little game with cars driving back and forth between to walls (colliders). My problem is that when a car is calling with it's second wall it's driving the wrong way e.g. Vector3(-1,0,0) instead of Vector3(1,0,0). I hope you can help me. Thanks in advance. Here's the script:

 var acc : float;
 
 private var speed : float;
 private var forward : Vector3 = Vector3(0,0,0);
 private var timeset : float;
 
 var hitSound: AudioClip;
 var driveSound : AudioClip;
 var gameover : AudioClip;
     
 function OnTriggerEnter(collision : Collider) { if (collision.gameObject.tag == "wallleft" && transform.position.z == -60)
 {    
     transform.position.z = transform.position.z-60;
     audio.loop = false;
     transform.Rotate(0,-180,0);
     timeset = Time.time;
     forward = Vector3(0,0,0);
     yield WaitForSeconds(2.0);
     forward = Vector3(-1,0,0);
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 else if (collision.gameObject.tag == "wallleft" && transform.position.z == -70)
 {
     transform.position.z = transform.position.z-40;
     audio.loop = false;
     transform.Rotate(0,-180,0);
     timeset = Time.time;
     forward = Vector3(0,0,0);
     yield WaitForSeconds(2.0);
     forward = Vector3(-1,0,0);
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 else if (collision.gameObject.tag == "wallleft" && transform.position.z == -80)
 {
     transform.position.z = transform.position.z-20;
     audio.loop = false;
     transform.Rotate(0,-180,0);
     timeset = Time.time;
     forward = Vector3(0,0,0);
     yield WaitForSeconds(2.0);
     forward = Vector3(-1,0,0);
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 else if (collision.gameObject.tag == "wallright" && transform.position.z == -120)
 {
     transform.position.z = transform.position.z+60;
     audio.loop = false;
     transform.Rotate(0,180,0);
     timeset = Time.time;
     forward = Vector3(0,0,0);
     yield WaitForSeconds(2.0);
     forward = Vector3(1,0,0);
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 else if (collision.gameObject.tag == "wallright" && transform.position.z == -110)
 {
     transform.position.z = transform.position.z+40;
     audio.loop = false;
     transform.Rotate(0,180,0);
     timeset = Time.time;
     forward = Vector3(0,0,0);
     yield WaitForSeconds(2.0);
     forward = Vector3(1,0,0);
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 else if (collision.gameObject.tag == "wallright" && transform.position.z == -100)
 {
     transform.position.z = transform.position.z+20;
     audio.loop = false;
     transform.Rotate(0,180,0);
     timeset = Time.time;
     forward = Vector3(0,0,0);
     yield WaitForSeconds(2.0);
     forward = Vector3(1,0,0);
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 }
 
 function Start() { if (transform.position.z == -100 || transform.position.z == -110 || transform.position.z == -120) // || = OR
 {
     forward = Vector3(1,0,0);
     timeset = Time.time;
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 else 
 {
     forward = Vector3(-1,0,0);
     timeset = Time.time;
     audio.clip = driveSound;
     audio.loop = true;
     audio.pitch = 1.0;
     audio.Play();
 }
 }
 
 function Update () {
 
     // Calculate speed from acceleration and time
     speed = acc * (Time.time-timeset);
         
     // Move the object forward along its x axis speed unit/second.
     transform.Translate(speed * forward * Time.deltaTime);    
 }