Parking barrier on an Arduino Uno

// Parking barrier: an HC-SR04, a traffic light module and an MG996R servo.
//
//   pins 9, 8     the HC-SR04's TRIG and ECHO
//   pins 6, 5, 4  the light's R, Y and G (resistors on its board)
//   pin 3         the MG996R's signal; the servo has its own 6 V supply,
//                 sharing ground with the Uno
//
// The barrier starts down and red. A car pulling up to within 20 cm turns
// the light yellow, lifts the arm to 90 degrees and turns it green. Once the
// lane has been clear (over 30 cm) for two seconds the light goes yellow,
// the arm comes down and it is red again. The gap between 20 and 30 cm keeps
// a car sitting right at the line from flapping the barrier.

#include <Servo.h>

const int TRIG = 9, ECHO = 8;
const int RED_PIN = 6, YELLOW_PIN = 5, GREEN_PIN = 4;
const int ARM = 3;

const long ARRIVE_CM = 20;
const long CLEAR_CM = 30;
const unsigned long CLEAR_MS = 2000;

Servo arm;
bool open = false;
unsigned long clearSince = 0;

void lights(bool r, bool y, bool g) {
  digitalWrite(RED_PIN, r ? HIGH : LOW);
  digitalWrite(YELLOW_PIN, y ? HIGH : LOW);
  digitalWrite(GREEN_PIN, g ? HIGH : LOW);
}

long distanceCm() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  unsigned long us = pulseIn(ECHO, HIGH, 30000UL);
  if (us == 0) return 400;
  return (long)(us / 58);
}

void swing(int from, int to) {
  int step = to > from ? 1 : -1;
  for (int a = from; a != to; a += step) {
    arm.write(a);
    delay(8);
  }
  arm.write(to);
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: parking barrier");
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(RED_PIN, OUTPUT);
  pinMode(YELLOW_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
  arm.attach(ARM);
  arm.write(0);
  lights(true, false, false);
  Serial.println("barrier down");
}

void loop() {
  long cm = distanceCm();
  if (!open && cm <= ARRIVE_CM) {
    Serial.print("car at ");
    Serial.print(cm);
    Serial.println(" cm: lifting");
    lights(false, true, false);
    swing(0, 90);
    lights(false, false, true);
    open = true;
    clearSince = millis();
    Serial.println("barrier up");
  } else if (open) {
    if (cm <= CLEAR_CM) {
      clearSince = millis();
    } else if (millis() - clearSince > CLEAR_MS) {
      Serial.println("lane clear: lowering");
      lights(false, true, false);
      swing(90, 0);
      lights(true, false, false);
      open = false;
      Serial.println("barrier down");
    }
  }
  delay(100);
}