codice:
<html>
<head>
<meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false"></script>
<script type="text/javascript">
  var directionsDisplay;
  var directionsService = new google.maps.DirectionsService();
  var map;

  function initialize() {
    var myOptions = {
      zoom: 13,
      center: new google.maps.LatLng(45.476744,9.20738),
      mapTypeId: google.maps.MapTypeId.ROADMAP
    }
    map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);

    directionsDisplay = new google.maps.DirectionsRenderer({
        'map': map,
        'preserveViewport': true,
        'draggable': true
    });
    directionsDisplay.setPanel(document.getElementById("directions_panel"));
  }

  function calcRoute(start) {
    var end = 'Corso Buenos Aires, 14, 20129 Milano';
    var request = {
        origin:start,
        destination:end,
        travelMode: google.maps.DirectionsTravelMode.DRIVING
    };
    directionsService.route(request, function(response, status) {
      if (status == google.maps.DirectionsStatus.OK) {
        directionsDisplay.setDirections(response);
      }
    });
  }


</script>
</head>
<body onload="initialize()">
<div id="map_canvas" style="float:left;width:70%;height:100%"></div>
<div style="float:right;width:30%;height:100%;overflow:auto">
  <input type="text" id="partenza"><input type="button" value="Calcola" onClick="calcRoute(document.getElementById('partenza').value)">
  <div id="directions_panel" style="width:100%"></div>
</div>
</body>
</html>
Ho modifcato questo esempio aggiungendo la possibilità di digitare la partenza