Realistic player movement?

Ok so I’m trying to make a some what more realistic movement system in my game, where walking backwards/strafing is slower than walking forwards

I have this down, the only problem is that when you go diagonal you get both the strafing and forward speed working at the same time meaning the player moves faster.

What would be the best way to combat this problem?

Here is what I have at the moment

    if(Input.GetButton("Forward")) {
		rigidbody.AddRelativeForce(Vector3.forward * forwardSpeed);
	if(Input.GetButton("Back")) {
		rigidbody.AddRelativeForce(Vector3.back * backSpeed);
	if(Input.GetButton("Left")) {
		rigidbody.AddRelativeForce(Vector3.left * strafeSpeed);
	if(Input.GetButton("Right")) {
		rigidbody.AddRelativeForce(Vector3.right * strafeSpeed);

What I would do is:

var forward : boolean;
var back : boolean;
var left : boolean;
var right : boolean;
var forwardSpeed : float;
var backSpeed : float;
var strafeSpeed : float;
var angleSpeed : float;
function Update() {
    forward = Input.GetButton( "Forward" );
    back = Input.GetButton( "Back" );
    left = Input.GetButton( "Left" );
    right = Input.GetButton( "Right" );
    if( forward && back || left && right ){
    if( forward ) {
        if( left ) {
            rigidbody.AddRelativeForce( Vector3( -1, 0, 1 ) * angleSpeed );
        else if( right ) {
            rigidbody.AddRelativeForce( Vector3( 1, 0, 1 ) * angleSpeed );
        else {
            rigidbody.AddRelativeForce(Vector3.forward * forwardSpeed);
    //do the same for back here
    else if( left ) {
         rigidbody.AddRelativeForce( -Vector3.right * strafeSpeed);
    //do the same for right

Also, I notice you using Vector3.back and Vector3.left, those don’t exist. You have Vector3.forward/right/up/one/zero which correspond to Vector3(0,0,1)/(1,0,0)/(0,1,0)/(1,1,1)/(0,0,0). So if you want left (which is (-1,0,0)) use -Vector3.right

I would just do the same checks as Joshua (if forward, check for left and right). And then take the average of your forward- and sideways-speed. When going diagonally backwards, just take backwards speed, or even a bit slower. As walking backwards and sideways is a bit harder then just backwards ;),I would calculate the average of the 2, when you are going diagonally. Diagonally backwards would have the same speed as backwards, or even slower.