fetchAPI

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <!-- <link rel="stylesheet" type="text/css" href="styles.css"> -->
</head>
<body>
  <select id="sel">
      <option> --場所を選ぶ-- </option>
      <option value="kyoto">京都</option>
      <option value="osaka">大阪</option>
      <option value="kobe">神戸</option>
  </select>

  <div id="mes">
    集合場所は<span id="pos"></span>
    集合時刻は<span id="time"></span>
  </div>
    <script>
      const url = "ajax_getData.php";
      let selid = document.getElementById('sel');

      selid.addEventListener('change', function(){
        let optval = selid.options[selid.selectedIndex].value;
        let param = new URLSearchParams();
        param.append("opt",optval);

        fetch(url, {
          method: "post",
          body: param,
        }).then(response => {
            if(response.ok){
              let promise = response.json();
              promise.then(data =>{
                document.getElementById('pos').innerHTML = data.position;
                document.getElementById('time').innerHTML = data.ap_time;
              })
            } else {
              alert("request failed");
            }
        })
      })
    </script>
</body>
</html>

なるほど。。。
ajaxの取得をtime intervalで数秒ごとに取るようにして表示すれば良いのか。
ほぼほぼロジックも出来たので背景処理のところだな