<ul id="traffic">
  <li><span></span></li>
  <li><span></span></li>
  <li><span></span></li>
</ul>

#traffic>li{
      display:block;
      }
    #traffic span{
      display:inline-block;
      width:50px;
      height:50px;
      background-color:gray;
      margin:5px;
      border-radius:50%;
      float:left;
    }
    #traffic.stop li:nth-child(1) span{
      background-color:yellow;
    }
    #traffic.wait li:nth-child(2) span{
      background-color:red;
    }
    #traffic.pass li:nth-child(3) span{
      background-color:green;
    }

const traffic = document.getElementById("traffic");
(function reset(){
  traffic.className = "wait";
  setTimeout(function(){
    traffic.className = "stop";
    setTimeout(function(){
      traffic.className = "pass";
      setTimeout(reset,2000);
    },2000);
  },2000);
})();