how to Increase speed of a car.?

I'm developing a car racing game. I've used following script to move car and increase speed gradually but problem I'm facing is car is moving at constant speed.Help me to maintain speed on key down event.

       var CurrentSpeed : float = 1;
       var RotationSpeed : float = 10.0;
       transform.Translate(Vector3.forward * Time.deltaTime * CurrentSpeed , Space.Self);
       if(CurrentSpeed < 0)
       {
       CurrentSpeed +=100* Time.deltaTime;
      } 
     else 
     {
      CurrentSpeed -=100* Time.deltaTime;
     }
     if(Input.GetKey("w"))
    {
     CurrentSpeed +=Time.deltaTime;
     transform.Translate(Vector3.back * Time.deltaTime * CurrentSpeed , Space.Self);
    }
    if(Input.GetKey("s"))
    {
     CurrentSpeed -= 10*Time.deltaTime;
         transform.Translate(Vector3.forward * Time.deltaTime * CurrentSpeed , Space.Self);
    }
    if(Input.GetKey(KeyCode.LeftArrow))
    {
     transform.Rotate(Vector3.up * Time.deltaTime * -RotationSpeed, Space.Self);
    }
  
     if(Input.GetKey(KeyCode.RightArrow)){
         transform.Rotate(Vector3.up * Time.deltaTime * RotationSpeed, Space.Self);
     }`