Implementing snake-like 2D controls and behavior

Hi, I'm totally new to Unity, and I'm trying to get the hang of it by creating a simple snake clone. So the controls I'm after are:

• Each fixed time frame, snake advances a fixed distance in the direction it's currently looking,

• left key changes the direction by -90 degrees around the z-axis,

• right key changes it by +90 degrees.

I've got some code that sort of does that, but occasionally it makes 2 turns after a key is pressed instead of 1. Any ideas how I might fix this are much appreciated!

 using UnityEngine;
 using System.Collections;
 
 public class PlayerController : MonoBehaviour {
     int ticks = 0;
     int tickThreshold = 30;
     float stepSize = 1.0f;
     Vector3 nextDirectionChange = Vector3.zero;
     
     void Start () {
         transform.forward = Vector3.right;
     }
     
     void FixedUpdate () {
         this.ticks++;
         if (this.ticks > this.tickThreshold) {
             transform.Rotate(this.nextDirectionChange);
             transform.position += transform.forward * stepSize;
             this.ticks = 0;
             this.nextDirectionChange = Vector3.zero;
         }
 
         if (Input.GetKey(KeyCode.LeftArrow)) {
             this.nextDirectionChange = Vector3.left * 90;
         }
         if (Input.GetKey(KeyCode.RightArrow)) {
             this.nextDirectionChange = Vector3.right * 90;
         }
     }
 }