const int trigPin = 3;    // Trigger pin of the ultrasonic sensor
const int echoPin = 4;   // Echo pin of the ultrasonic sensor
const int redPin =5;    // Red LED pin
const int yellowPin = 6; // Yellow LED pin
const int greenPin = 7;  // Green LED pin

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(redPin, OUTPUT);
  pinMode(yellowPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
}

void loop() {
  // Normal traffic system
  normalTraffic();

  // Check for emergency condition
  if (checkEmergency()) {
    // Emergency traffic condition
    emergencyTraffic();
  }
}

void normalTraffic() {
  // Red LED on for 3 seconds
  digitalWrite(redPin, HIGH);
  delay(3000);

  // Red LED off
  digitalWrite(redPin, LOW);

  // Blink yellow LED twice for 0.5 seconds each
  for (int i = 0; i<3; i++) {
    digitalWrite(yellowPin, HIGH);
    delay(500);
    digitalWrite(yellowPin, LOW);
    delay(500);
  }

  // Green LED on for 5 seconds
  digitalWrite(greenPin, HIGH);
  delay(5000);

  // Green LED off
  digitalWrite(greenPin, LOW);
}

bool checkEmergency() {
  // Read distance from ultrasonic sensor
  long duration, distance;
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = duration * 0.034 / 2;

  // Check if distance is less than 15 cm
  if (distance < 15) {
    return true; // Emergency condition detected
  } else {
    return false; // No emergency condition
  }
}

void emergencyTraffic() {
  // Turn on green LED immediately
  digitalWrite(greenPin, HIGH);

  // Loop until obstacle is removed
  while (checkEmergency()) {
    // Continue monitoring for obstacle
  }

  // Turn off green LED when obstacle is removed
  digitalWrite(greenPin, LOW);
}