codice:
<html>
<head>
<meta name="viewport" content="initial-scale=1.0, user-scalable=no"/>
<meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
<title>Google Maps API v3: Directions</title>
<style type="text/css">
html, body { margin:0; padding:0; width:100%; height:100%; }
body { background:#FFFFFF; color:#000000; font-family:Arial, Helvetica, sans-serif; font-size:12px; line-height:150%; text-align:center;}
#map { width:100%; height:95%; }
input { width:250px; }
</style>
<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false"></script>
<script type="text/javascript">
var directionDisplay;
var directionsService = new google.maps.DirectionsService();
var map;

function initialize() {
  
    calcRoute();
	directionsDisplay = new google.maps.DirectionsRenderer();
    var myOptions = { 
		zoom:7,
      	mapTypeId: google.maps.MapTypeId.ROADMAP
    }
    
	map = new google.maps.Map(document.getElementById("map"), myOptions);
    
	directionsDisplay.setMap(map);
	
	document.getElementById("submit").onclick = function() {
		calcRoute();
	}
	
}
  
  function calcRoute() {
    var partenza = document.getElementById("partenza").value;
    var arrivo = document.getElementById("arrivo").value;
    var request = {
        origin:partenza, 
        destination:arrivo,
        travelMode: google.maps.DirectionsTravelMode.DRIVING,
		provideRouteAlternatives: true
    };
    directionsService.route(request, function(response, status) {
      if (status == google.maps.DirectionsStatus.OK) {
        directionsDisplay.setDirections(response);
      }
    });
  }
window.onload = initialize;
</script>
</head>
<body>
<div>
Partenza: 
<input type="text" id="partenza" value="Via del Portonaccio - Roma">
Arrivo: 
<input type="text" id="arrivo" value="Via Zamboni - Bologna">
<input id="submit" type="button" value="trova percorso sulla mappa">
</div>
<div id="map"></div>
</body>
</html>
Comunque, come dicevi tu, ho aggiunto quel parametro ma non cambia nulla sulla mappa