How to increase the acceleration of the object and affect an emission materials brightness based on it

Hello all; I am creating a game like cubefield and tron legacy and I have came across a bump in the road. I dont know how to add acceleration to this movement script. Let alone correlate that with the emission output of a material. What I am trying to do is if the w key is pressed it will accelerate to a certain max speed and if it is let go it will slowly de-accelerate. I want the emission material to increase its brightness based on the objects speed and decrease also based on its speed. I just need help with those 2 things. Here is my movement script:

  [Tooltip("How fast the character should move with no input, in world units per second")]
     public float autoMoveSpeed = 40.0f;
     [Tooltip("The direction the character should move with no input")]
     public Vector3 autoMoveDirection = Vector3.right;
     // Enforce that this is always a unit direction vector, so it doesn't bias our speed.
     void OnValidate() { autoMoveDirection = autoMoveDirection.normalized; }
 
     [Tooltip("How much the player can affect the speed in the direction of auto movement")]
     public float parallelMoveSpeed = 50f;
     [Tooltip("How fast the player can affect the speed perpendicular to the auto movement")]
     public float perpendicularMoveSpeed = 70f;
 
     [Tooltip("How many points to earn per second with no input")]
     public float scorePerSecondAtDefaultSpeed = 10f;
     [Tooltip("How many extra points to earn per second for every extra unit of speed")]
     public float extraScorePerVelocityUnit = 1.0f;
 
     // Track how much "loose change" points we've accumulated.
     float _scoreAccumulator = 0f;
 
     void Update()
     {
         // Capture the player's input vector.
         Vector3 input = new Vector3(
                             Input.GetAxis("Horizontal"),
                             0,
                             Input.GetAxis("Vertical")
                         );
 
         // Clamp the input to make sure we don't move faster on diagonals.
         input = Vector3.ClampMagnitude(input, 1.0f);
 
         // Extract the portion of the player's input parallel to the auto movement.
         float parallelComponent = Vector3.Dot(autoMoveDirection, input);
 
         // Leaving the component perpendicular to the auto movement.
         Vector3 perpendicularComponent = (input - parallelComponent * autoMoveDirection);
 
         // Compute how much to increase or decrease our auto movement based on our input.
         float extraVelocity = parallelComponent * parallelMoveSpeed;
 
 
         // Compute our net velocity, combining auto, parallel, and perpendicular movement.
         Vector3 velocity = autoMoveDirection * (autoMoveSpeed + extraVelocity)
                          + perpendicularComponent * perpendicularMoveSpeed;
 
 
         // Translate us according to that velocity and the elapsed time.
         transform.Translate(velocity * Time.deltaTime, Space.Self);
 
         // Scoring:
         // Compute our increased (or reduced) scoring rate based on our forward velocity.
         float extraScore = extraVelocity * extraScorePerVelocityUnit;
 
         // Increase our score accumulator based on our current earning rate and time passed.
         _scoreAccumulator += (scorePerSecondAtDefaultSpeed + extraScore) * Time.deltaTime;
 
         // Whenever we earn a whole point, send it to the score display as an integer.
         int scoreUnits = Mathf.FloorToInt(_scoreAccumulator);
         scorecript.scoreValue += scoreUnits;
         // And decrement our accumulator to store just the "spare change".
         _scoreAccumulator -= scoreUnits;
     }