Finite state machine

traffic-lights.el

(defun traffic-lights ()
  (let ((state 'red))
    (while t
      (cond
       ((eq state 'red)
        (message "Red")
        (sleep-for 1)
        (setq state 'red+yellow))
       ((eq state 'red+yellow)
        (message "Red+Yellow")
        (sleep-for 0.5)
        (setq state 'green))
       ((eq state 'green)
        (message "Green")
        (sleep-for 1)
        (setq state 'yellow))
       ((eq state 'yellow)
        (message "Yellow")
        (sleep-for 1)
        (setq state 'red))))))

(traffic-lights)