const int redVertical = 9;
const int yellowVertical = 10;
const int greenVertical = 8;

const int redHorizontal = 12;
const int yellowHorizontal = 13;
const int greenHorizontal = 11;

const int traffic_light_timer = 15 * 1000;
const int traffic_yellow_timer = 3 * 1000;

void light_vertical(){
  digitalWrite(redVertical, LOW);
  digitalWrite(greenVertical, HIGH);
  delay(traffic_light_timer);
  digitalWrite(yellowVertical, HIGH);
  digitalWrite(greenVertical, LOW);
  delay(traffic_yellow_timer);
  digitalWrite(yellowVertical, LOW);
  digitalWrite(redVertical, HIGH);
}

void light_horizontal(){
  digitalWrite(redHorizontal, LOW);
  digitalWrite(greenHorizontal, HIGH);
  delay(traffic_light_timer);
  digitalWrite(yellowHorizontal, HIGH);
  digitalWrite(greenHorizontal, LOW);
  delay(traffic_yellow_timer);
  digitalWrite(yellowHorizontal, LOW);
  digitalWrite(redHorizontal, HIGH);
}

void setup() {
  pinMode(redHorizontal, OUTPUT);
  pinMode(yellowHorizontal, OUTPUT);
  pinMode(greenHorizontal, OUTPUT);
  pinMode(redVertical, OUTPUT);
  pinMode(yellowVertical, OUTPUT);
  pinMode(greenVertical, OUTPUT);
  digitalWrite(redHorizontal, HIGH);
  digitalWrite(redVertical, HIGH);
}

void loop() {
  light_horizontal();
  light_vertical();
}