I keep getting this error on my array however i don't think it is out of bounds because i have specified its length. the error will give me a number but for my program they aren't out of bounds.

final int DASH_LENGTH = 10;
final int LANE_HEIGHT = 30;
final int GAP = 3;
final int DIFFICULTY = (int)random(1,6); //keep between 1 and 5
final int N_CARS = 6 + DIFFICULTY;
final int CAR_HEIGHT = 24;
final int FROG_SIZE = 24;
int score = 0;

Car[] cars;
Frog frog;

void setup() {
  size(600,480);
  background(255);
  frog = new Frog();
  cars = new Car[6 + DIFFICULTY];
  for (int i=0; i<cars.length; i++) {
    int carLength = (int)random(40,101);
    cars[i] = new Car(carLength, CAR_HEIGHT, 15 + i*30);
  }
}

void draw() {
  background(255);
  lanes();
  for(int i = 0; i <= cars.length; i++) {
    cars[i].displayTheCarMove();     /*THIS IS WHERE I GET THE ERROR. BUT ABOVE THERE IS A SIMILAR LINE OF CODE AND THAT WORKED FINE. */
  }
  frog.display();
}

void keyPressed() {
  frog.move();
}

class Car {
  float cWidth, cHeight;
  float centerX, centerY;
  float speed;
  int shade;
 
  Car(float cL, float cH, float lane) {
    cWidth = cL;
    cHeight = cH;
    centerX = random(cL/2, width - cL/2);
    centerY = constrain(centerY, 3 + cH/2, (6 + DIFFICULTY)*LANE_HEIGHT);
  }
  void displayTheCarMove() {
    display();
    carMove();
  }
 
  void carMove() {
    centerX += speed;
    if((centerX <= cWidth/2) || (centerX >= width - cWidth / 2)) {
      speed *= -1;
    }
  }
 
  void display() {
    rectMode(CENTER);
    noStroke();
    fill((int)random(201));
    rect(centerX, centerY, cWidth, cHeight);
  }
}

Thank you in advance.