Obstacle Avoiding Robot using Arduino

int trigPin = 9;      // trig pin of HC-SR04
int echoPin = 10;     // Echo pin of HC-SR04

int left_1 = 4;       //Reverse motion of Left motor
int left_2 = 5;       //Forward motion of Left motor
int right_1 = 6;      //Reverse motion of Right motor
int right_2 = 7;      //Forward motion of Right motor

long duration, distance;

void setup() {
  Serial.begin(9600);
  pinMode(left_1, OUTPUT);
  pinMode(left_2, OUTPUT);
  pinMode(right_1, OUTPUT);
  pinMode(right_2, OUTPUT);

  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {

  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  duration = pulseIn(echoPin, HIGH);
  distance = duration / 58.0;
  delay(10);

  if (distance > 19)
  {
    digitalWrite(left_1, HIGH);                    // move forward
    digitalWrite(left_2, LOW);
    digitalWrite(right_1, HIGH);
    digitalWrite(right_2, LOW);
  }

  if (distance < 18)
  {
    digitalWrite(left_1, LOW);   //stop
    digitalWrite(left_2, LOW);
    digitalWrite(right_1, LOW);
    digitalWrite(right_2, LOW);
    delay(500);
    digitalWrite(left_1, LOW);
    digitalWrite(left_2, HIGH);
    digitalWrite(right_1, LOW);
    digitalWrite(right_2, HIGH);
    delay(500);
    digitalWrite(left_1, LOW);   //stop
    digitalWrite(left_2, LOW);
    digitalWrite(right_1, LOW);
    digitalWrite(right_2, LOW);
    delay(100);
    digitalWrite(left_1, HIGH);
    digitalWrite(left_2, LOW);
    digitalWrite(right_1, LOW);
    digitalWrite(right_2, LOW);
    delay(500);
  }
}