Endless Runner game: Turn Corners

I use the following codes in Update() to move the character, Runner, a GameObject :

rigidbody.AddForce(new Vector3 (speed, 0, 0), ForceMode.Acceleration);

I use the following codes in Update() to change the direction of the Runner GameObject :

if (Input.GetKey(KeyCode.LeftArrow)) {
  Quaternion target = Quaternion.Euler(0, 90, 0);
  gameObject.transform.rotation = Quaternion.Slerp(transform.rotation, target, Time.deltaTime * smooth);

The Runner can rotate 90degree, but its movement direction does not change. Also, maximum rotation degree is 90degree, why? Did I misunderstand the use of the function Quaternion.Slerp() ?

Also, the Runner still runs at original direction, how to make the Runner change direction ( following the rotation ) ?

The rotation only goes up to 90 degrees because that’s what you are using as target. You need to add 90 degrees to the target every time you turn.

As for why the runner always runs in the same direction, that’s because you are always applying the same force direction, (speed, 0, 0). You can change it to be always directed to the character’s forward direction like this:

rigidbody.AddForce(speed * transform.forward, ForceMode.Acceleration);