Código

//INICIA LA CLASE TRAFICO

package semaforo;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.util.Random;

import javax.swing.JFrame;

public class Trafico extends Thread {

	private int cola[] = {0, 0, 0, 0, 0, 0};
	private int posX[] = {620, 620, 330, 330, 530, 580};
	private int posY[] = {90, 130, 180, 220, 300, 300};
	private Graphics gr;
	private Color COLORS[] = {Color.green, Color.red, Color.blue};
	private Random r;
	private Datos data;
	private BufferedImage clr;
	private JFrame ventana;

	public Trafico(Datos data, BufferedImage cl) {
		this.gr = data.getGraphics();
		this.data = data;
		this.clr = cl;
		this.ventana = data.getVentana();
		r = new Random();
	}

	public void run() {
		try {
			int t, tiempo = 1000;
			while (true) {
				for(int sem = 0; sem < 4; sem++) {
					data.pausa();
					System.out.println(" - -  Semaforo " + Integer.toString(sem + 1) );
					t = optimizar(sem);
					for(int i = 0; i < t; i++) {
						autos();
						avanza(sem);
						debug();
						sleep(tiempo);
					}
				data.go();
				}
			}
		} catch(Exception e){
			System.out.println("-- ERROR --");
		}
	}

	private int optimizar(int v) {
		int a;
		if (v == 0) {
			a = Math.max(cola[0] / 2, Math.max(cola[1], cola[5]) );
		}
		else if (v == 1) {
			a = Math.max(cola[0], cola[2] / 3 );
		}
		else if (v == 2) {
			a = Math.max(cola[2], cola[3] / 3 );
		}
		else {
			a = Math.max(cola[4], Math.max(cola[5], cola[3]) );
		}
		a = Math.max(a+3, 6);
		if(a > 15){
			a = 18;
		}
		return a;
	}

	private void debug() {
		for (int i= 0; i < 6; i++) {
			System.out.print(cola[i] + " ");
		}
		System.out.println();
	}

	private void avanza(int v) {
		if (v == 0) {
			cola[0] = salir(cola[0], 0);
			cola[1] = salir(cola[1], 1);
			cola[5] = salir(cola[5], 5);
		}
		else if (v == 1) {
			cola[0] = salir(cola[0], 0);
			cola[2] = salir(cola[2], 2);
		}
		else if(v == 2) {
			cola[2] = salir(cola[2], 2);
			cola[3] = salir(cola[3], 3);
		}
		else {
			cola[3] = salir(cola[3], 3);
			cola[4] = salir(cola[4], 4);
			cola[5] = salir(cola[5], 5);
		}
	}

	private int salir(int valor, int sverde) {
		if(valor > 0) {
			valor -= 1;
			clear(sverde);
		}
		return valor;
	}

	private void clear(int c) {
		int a = 0, b = 0;
		if (c/2 == 0) {
			a = 20;
		}
		else if (c/2 == 1) {
			a = -20;
		}
		else {
			b = 20;
		}
		posX[c] -= a;
		posY[c] -= b;
		gr.drawImage(clr, posX[c], posY[c], ventana);
	}

	private void autos() {
		int cantidad, i;
		for (int j = 0; j < 2; j++) {
			cantidad = r.nextInt(1) + 1;
			i = r.nextInt(6);
			cola[i] += cantidad;
			paint(cantidad, i);
		}
	}

	private void paint(int n, int c) {
		int a = 0, b = 0;
		gr.setColor(COLORS[c/2]);
		if (c/2 == 0) {
			a = 20;
		}
		else if (c/2 == 1) {
			a = -20;
		}
		else {
			b = 20;
		}
		for(int i = 0; i< n; i++) {
			gr.fillRect(posX[c], posY[c], 15, 15);
			posX[c] += a;
			posY[c] += b;
		}
	}

}

//TERMINA LA CLASE TRAFICO


//INICIA LA CLASE MAIN

package semaforo;

public class Main{

	public static void main(String[] args) {
		Datos data = new Datos();
    	Panel ambiente = new Panel(data);
    	Trafico autos = new Trafico(data, ambiente.getClear() );
    	ambiente.start();
    	autos.start();
    }
}

//TERMINA LA CLASE MAIN

//INICIA LA CLASE PANEL

package semaforo;

import java.awt.*;

import javax.swing.*;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;


public class Panel extends Thread {

	private JFrame ventana;
	private String sem1, sem2, sem3, sem4;
    private BufferedImage mapa, s1, s2, s3, s4, clear;
    int tiempoespera;
	private Datos data;

public Panel(Datos data){
	this.data = data;

	cargarimagen(1);
	data.getGraphics().drawImage(mapa,0,0, ventana);

	}

public void run(){
	try {
		System.out.println("arranca hilo");
		while(true) {
			for(int i=1; i<=4; i++) {
				cargarimagen(i);
				paint(data.getGraphics());
				System.out.println("semaforo " + i);
				data.go();
				data.pausa();
			}
		}

	} catch(Exception e) {
		e.printStackTrace();
	}
}

public void cargarimagen(int secuencia){
	int valor = secuencia;
	if(valor == 1){
		 sem1 = "imagenes/s1rv.png"; // arriba
		 sem2 = "imagenes/s2r.png";  // peaton
		 sem3 = "imagenes/s3rr.png"; // derecha
		 sem4 = "imagenes/s4vv.png"; // izq
		}
	else if(valor == 2){
		 sem1 = "imagenes/s1rr.png";
		 sem2 = "imagenes/s2v.png";
		 sem3 = "imagenes/s3vr.png";
		 sem4 = "imagenes/s4vr.png";
		}
	else if(valor == 3){
		sem1 = "imagenes/s1rr.png";
		sem2 = "imagenes/s2r.png";
		sem3 = "imagenes/s3vv.png";
		sem4 = "imagenes/s4vr.png";
	}
	else {
		sem1 = "imagenes/s1vv.png";
		sem2 = "imagenes/s2r.png";
		sem3 = "imagenes/s3rv.png";
		sem4 = "imagenes/s4rr.png";
	}
	try {
		mapa = ImageIO.read(getClass().getClassLoader().getResource("imagenes/fondo.png"));
		s1 = ImageIO.read(getClass().getClassLoader().getResource(sem1));
		s2 = ImageIO.read(getClass().getClassLoader().getResource(sem2));
	    s3 = ImageIO.read(getClass().getClassLoader().getResource(sem3));
	    s4 = ImageIO.read(getClass().getClassLoader().getResource(sem4));
	    clear =  ImageIO.read(getClass().getClassLoader().getResource("imagenes/clear.png"));
	} catch(Exception e) {
		System.out.println("no se pudo cargar las imagen");
	}
    }

public void paint(Graphics gr){
	gr.drawImage(s1, 520, 30, ventana);
	gr.drawImage(s2, 630, 290, ventana);
	gr.drawImage(s3, 630, 175, ventana);
	gr.drawImage(s4, 300, 85, ventana);
}

public BufferedImage getClear() {
	return clear;
}

public JFrame getVentana() {
	return ventana;
}

}

//TERMINA LA CLASE PANEL

//INICIA LA CLASE DATOS

package semaforo;

import java.awt.Container;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;


public class Datos {

	private JFrame ventana;
	private JPanel panel;

	public Datos() {
		ventana = new JFrame("Semaforo");
		ventana.setSize(950, 650);
		ventana.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		Container contenido = ventana.getContentPane();

		panel = new JPanel();
		panel.setSize(950, 650);

		contenido.add(panel);
		ventana.setVisible(true);
	}

	public synchronized void pausa() {
		try {
			wait();
		} catch(Exception e) {
			System.out.println("Error");
		}
	}

	public synchronized void go() {
		notify();
	}

	public Graphics getGraphics() {
		return panel.getGraphics();
	}

	public JFrame getVentana() {
		return ventana;
	}

}

//TERMINA LA CLASE DATOS