int  	x, y;
float 	speed;

void setup() {
  size(600,300);
  x = 0;
  y = 300;
  speed = 0;
}

void draw() {
  background(255,255,255);
  drawCar(x);
  x = int(x + speed);
  if (x+100 > width) 
    noLoop();
  if (x+50 < width/2) 
    speed = speed + 0.10;
  else 
    speed = speed - 0.10;
  speed = max(0, speed);
}

void drawCar(int x) {
  // Draw the body
  fill(150,150,150);  // gray
  rect(x, y-30, 100, 20);
  quad(x+20,y-30,x+30,y-45,x+55,y-45,x+70,y-30);
  
  // Draw the wheels
  fill(0,0,0);  // black
  ellipse(x+20, y-10, 20, 20);
  ellipse(x+75, y-10, 20, 20);
  fill(255,255,255); // white
  ellipse(x+20, y-10, 10, 10);
  ellipse(x+75, y-10, 10, 10);
}