How to avoid movement speed stacking when multiple keys are pressed?

Question

I've started a new game which requires no mouse, thus leaving the movement up to the keyboard. I have tried to incorporate 8 directions; up, left, right, up-right and so on. However when I press more than one arrow key, the movement speed stacks (http://gfycat.com/CircularBewitchedBarebirdbat). How could I counteract this?

Here is relevant part of my code:

var speed : int = 5;

function Update () {
    if (Input.GetKey(KeyCode.UpArrow)) {
        transform.Translate(Vector3.forward * speed * Time.deltaTime);
    } else if (Input.GetKey(KeyCode.UpArrow) && Input.GetKey(KeyCode.RightArrow)) {
        transform.Translate(Vector3.forward * speed * Time.deltaTime);
    } else if (Input.GetKey(KeyCode.UpArrow) && Input.GetKey(KeyCode.LeftArrow)) {
        transform.rotation = Quaternion.AngleAxis(315, Vector3.up);
    }
    if (Input.GetKey(KeyCode.DownArrow)) {
        transform.Translate(Vector3.forward * speed * Time.deltaTime);
    }
}

Answer 1

Separate your direction selection code from actual movement code.

1. Choose Direction by checking which keys are pressed. Store it as a unit (normalized) vector.
2. Multiply your Direction with Speed and with DeltaTime.
3. Apply resulting transform to your object/camera.

Answer 2

You need to take the sum of the directions, normalize that, then multiply by the speed.

I tangentially answered this as part of my response to Preventing diagonal movement

Specifically:

velX = 0;
velY = 0;

if(keyLeft) velX += -1;
if(keyRight) velX += 1;
if(keyUp) velY += -1;
if(keyDown) velY += 1;

// Normalize to prevent high speed diagonals
length = sqrt((velX * velX ) + (velY * velY ));
if (length != 0)
{
    velX /= length;
    velY /= length;
}

velX *= speed;
velY *= speed;

Answer 3

The "normalized direction vector" is how this task is usually approached, and how I often do it, but lately I've simply been clamping the resulting movement vector. It usually achieves the same end result and the code is a lot simpler:

var moveSpeed = 6.0f;
function Update() {
  var movement = Vector3.zero;
  movement.x = Input.GetAxis("Horizontal") * moveSpeed;
  movement.z = Input.GetAxis("Vertical") * moveSpeed;
  movement = Vector3.ClampMagnitude(movement, moveSpeed);
  movement *= Time.deltaTime;
  transform.Translate(movement);
}

Simpler code is almost always better :E