Arduino randomly freezes during runtime?

I'm currently trying to create a traffic simulation project on Arduino, where it can change lights, open a gate, and turn on street lamps based on light. If anybody could help me diagnose why my arduino is behaving this way, that'd be extremely appreciated. Here is my code, if it helps you understand.

//Libraries Used
#include<Servo.h>


//Variable Declaration
int red1 = 10;
int yellow1 = 9;
int green1 = 8;
int red2 = 13;
int yellow2 = 12;
int green2 = 11;
int debug = 3;
int PIR = 5;

int button1 = 2;
int buttonState = 0;

int light = 0;

unsigned long previousTime = 0;
unsigned long currentTime = 0;
int currentState = 0;

int currentPIR = 0;
int pirState = LOW;

Servo servo;

void setup()
{
  pinMode(red1, OUTPUT);
  pinMode(yellow1, OUTPUT);
  pinMode(green1, OUTPUT);
  pinMode(red2, OUTPUT);
  pinMode(yellow2, OUTPUT);
  pinMode(green2, OUTPUT);
  pinMode(debug, OUTPUT);

  pinMode(PIR, INPUT);

  pinMode(button1, INPUT);

  servo.attach(4);
  servo.write(90);
}

void loop()
{

  currentPIR = digitalRead(PIR);
  if(currentPIR == 1){
    servo.write(180);
  }else{
    servo.write(90);
  }


  buttonState = digitalRead(2);
  light = analogRead(A0);

  if(light > 100){
    digitalWrite(debug, HIGH); 
  }else{
    digitalWrite(debug, LOW);
  }

  if(buttonState == HIGH){
    if(currentState == 3){
      digitalWrite(debug, HIGH);
      currentState++;
    }
  }
  //Using millis() instead of delay(), so program doesn't pause completely.
  currentTime = millis();
  changeLights();
}

void changeLights(){
  if(currentState == 0){
    //Red, Green
    digitalWrite(red2, LOW);
    digitalWrite(red1, HIGH);
    digitalWrite(green2, HIGH);
    asyncDelay(4000);
  }
  else if(currentState == 1){
    //Red, Yellow
    digitalWrite(green2, LOW);
    digitalWrite(yellow2, HIGH);
    asyncDelay(2000);
  }
  else if(currentState == 2){
    //Red, Red
    digitalWrite(yellow2, LOW);
    digitalWrite(red2, HIGH);
    asyncDelay(750);
  }
  else if(currentState == 3){
    //Green, Red
    digitalWrite(red1, LOW);
    digitalWrite(green1, HIGH);
    asyncDelay(4000);
  }
  else if(currentState == 4){
    //Yellow, Red
    digitalWrite(green1, LOW);
    digitalWrite(yellow1, HIGH);
    asyncDelay(2000);
  }
  else if(currentState == 5){
    //Red, Red
    digitalWrite(yellow1, LOW);
    digitalWrite(red2, HIGH);
    asyncDelay(750);
  }
}

void asyncDelay(int delayTime){
  if(currentTime - previousTime == delayTime){
    currentState++; 
    previousTime = currentTime;
    if(currentState == 6){
      currentState = 0;
    }
  }
}