int redled = 8;
int yellowled = 7;
int greenled = 6;

const int trigPin = 9;
const int echoPin = 10;

long duration;
int distance;

void setup() {
pinMode(redled, OUTPUT);
pinMode(yellowled, OUTPUT);
pinMode(greenled, OUTPUT);
pinMode(trigPin, OUTPUT); // Sets the trigPin as an Output
pinMode(echoPin, INPUT); // Sets the echoPin as an Input
Serial.begin(9600); // Starts the serial communication
}
void loop() {
// Clears the trigPin
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
// Sets the trigPin on HIGH state for 10 micro seconds
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
// Reads the echoPin, returns the sound wave travel time in microseconds
duration = pulseIn(echoPin, HIGH);
// Calculating the distance
distance= duration*0.034/2;
Serial.print("Distance: ");
Serial.println(distance);


if (distance <= 30){ //this
  digitalWrite(redled, LOW);
  digitalWrite(yellowled, LOW);
  digitalWrite(greenled, HIGH);
}

if (distance <= 20){ //this
  digitalWrite(redled, LOW);
  digitalWrite(yellowled, HIGH);
  digitalWrite(greenled, LOW);
}

if (distance <= 10){ //this
  digitalWrite(redled, HIGH);
  digitalWrite(yellowled, LOW);
  digitalWrite(greenled, LOW);
}

}